Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct word for each of the blanks.
Air  pollution  occurs  when  wastes  dirty  the  air.  People  produce  most  of  the  (1)........that cause air pollution. Such wastes can be in the form  of  gases  or  particles  of  solid  or  liquid matter. These substances result chiefly from burning (2).............. to power motor vehicles and to heat buildings. Industrial processes and the (3)...............of garbage also contribute to air pollution. Natural pollutants (impurities) include dust, pollen, soil particles, and naturally occurring gases.
The rapid growth of population and industry and the increasing use of automobiles and airplanes   have made air  pollution a (4)............. problem. The air we breathe has become so filled with  pollutants that  it  can cause (5)...........problems. Polluted air also harms plants, animals, building materials, and fabrics. In addition, it causes damage by altering the earth’s atmosphere.
(3).................................

Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions.
Christina and James met in college and have been dating for more than five years. For the  past two years, they have been living together in a  condo  they  purchased jointly. While Christina and James were confident in their decision to enter into a commitment like a  20-year mortgage,  they are unsure if they want to enter into marriage. The couple had many discussions about marriage and decided that it just did not seem necessary. Wasn't it only a piece of  paper? And  didn't half of all marriages end in divorce?
Neither Christina nor James had seen much success with marriage while growing up. Christina was raised by a single mother. Her parents never married, and her father has had little contact with the family since she was a toddler, Christina and her mother lived with her maternal grandmother, who often served as a surrogate parent. James grew up in  a  two-parent household  until age seven, when his parents divorced. He lived with his mother for a  few years, and then    later with his mother and her boyfriend until he left for college. James remained close with his  father who remarried and had a baby with his new wife.
Recently, Christina and James have been thinking about having children and the subject of marriage has resurfaced. Christina likes the idea of her children growing up in a traditional  family; while James is concerned about possible marital problems down the road and negative consequences for the children should that occur. When they shared these concerns with their parents, James's mom was adamant that the couple should get married. Despite having been divorced and having a live-in boyfriend of 15  years, she believes that children are better off when  their parents are married. Christina's mom believes that the couple should do whatever they want but adds that it would "be nice" if they wed. Christina and James's friends told them, married or not married; they would still be a family.

Which of the following could best replace the word "traditional" ?

Read the following passage and mark A, B, C, or D to indicate the correct answer to each of the blanks.
                           EXAM ADVICE
In Part Three of the Speaking Section you work together with a partner. You have to do a (16)...... task which usually lasts about 3 minutes. One possible task is “problem solving”, which means you have to look at some (17).......... information and then discuss the problem with your partner. You may be shown photos, drawings, diagrams, maps, plans, advertisements or computer graphics and it  is (18)......... that you study them carefully. If necessary, check you know exactly what to do by politely asking the examiner to repeat the instruction or make them clearer.
While you are doing the task, the examiner will probably say very little and you should ask your partner questions and make (19)..........if he or she is not saying much. If either of you have any real difficulties, the examiner may decide to step in and help. Normally, however, you will find plenty to say, which helps the assessor to give you a fair mark. This mark depends on your success in doing the task by competing with your partner, which includes taking (20)............... in giving opinions and replying appropriately, although in the end it may be possible to “agree to disagree”.
(17)............................

Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions:
Every summer, when the results of university entrance exam come out, many newspaper stories are published about students who are top-scorers across the country. Most portray students as hard- working, studious, smart and, generally, from low-income families. They are often considered heroes or heroines by their families, communes, villages and communities, And they symbolise the efforts made to lift them, and their relatives, out of poverty. The students are often too poor to attend any extra-classes, which make their achievements more illustrious and more newsworthy. While everyone should applaud the students for their admirable efforts, putting too much emphasis on success generates some difficult questions.
If other students look up to them as models, of course it's great. However, in a way, it contributes to society's attitude that getting into university is the only way to succeed. For those  who fail, their lives are over. It should be noted that about 1.3 million high school students take    part in the annual university entrance exams and only about 300,000 of them pass. What's about the hundreds of thousands who fail? Should we demand more stories about those who fail the exam  but succeed in life or about those who quit university education at some level and do something else unconventional?
"I personally think that it's not about you scoring top in an entrance exam or get even into Harvard. It's about what you do for the rest of your life," said Tran Nguyen Le Van, 29. He is the founder of a website, vexere.com, that passengers can use to book bus tickets online and receive tickets via SMS. His business also arranges online tickets via mobile phones and email. Van dropped out of his MBA at the Thunderbird School of Global Management in Arizona in the United States. His story has caught the attention of many newspapers and he believes more coverage should be given to the youngsters who can be role-models in the start-up community. Getting into university, even with honours, is just the beginning. We applaud them and their efforts and obviously that can give them motivation to do better in life. However, success requires more than just scores," Van said. Van once told a newspaper that his inspiration also came from among the world's most famous drop-outs, such as Mark Zuckerberg of Facebook or Bill Gates who also dropped out of Harvard University.
Alarming statistics about unemployment continues to plague us. As many as 162,000 people with some kind of degree cannot find work, according to Labour Ministry's statistics this month. An emphasis on getting into university does not inspire students who want to try alternative options. At the same time, the Ministry of Education and Training is still pondering on how to reform our exam system, which emphasises theories, but offers little to develop critical thinking or practice. Vu Thi Phuong Anh, former head of the Centre for Education Testing and Quality Assessment at Vietnam National University in Ho Chi Minh City said the media should also monitor student successes after graduation. She agreed there were many success stories about young people, but added that it was imbalanced if students taking unconventional paths were not also encouraged.
Vietnam is, more than ever, in desperate need of those who think outside the box. Time for us to recognise talent, no matter where it comes from or how.

The word "them" in paragraph 3  refers to    _.

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions:
The technology of the North American colonies did not differ strikingly from that of Europe, but in one respect, the colonists enjoyed a great advantage. Especially by comparison with Britain, Americans had a wonderfully plentiful supply of wood.
The first colonists did not, as many people imagine, find an entire continent covered by a climax forest. Even along the Atlantic seaboard, the forest was broken at many points. Nevertheless, all sorts of fine trees abounded, and through the early colonial period, those who pushed westward encountered new forests. By the end of the colonial era, the price of wood had risen slightly in eastern cities, but wood was still extremely abundant.
The availability of wood brought advantages that have seldom been appreciated. Wood was a foundation of the economy. Houses and all manner of buildings were made of wood to a degree unknown in Britain. Secondly, wood was used as fuel for heating and cooking. Thirdly, it was used as the source of important industrial compounds, such as potash, an industrial alkali; charcoal, a component of gunpowder; and tannic acid, used for tanning leather.
The supply of wood conferred advantages but had some negative aspects as well. Iron at that time was produced by heating iron ore with charcoal. Because Britain was so stripped of trees, she was unable to exploit her rich iron mines. But the American colonies had both iron ore and wood; iron production was encouraged and became successful. However, when Britain developed coke smelting, the Colonies did not follow suit because they had plenty of wood and besides, charcoal iron was stronger than coke iron. Coke smelting led to technologic innovations and was linked to the emergence of the Industrial Revolution. In the early nineteenth century, the former colonies lagged behind Britain in industrial development because their supply of wood led them to cling to charcoal iron.

What can be inferred about houses in Britain during the period written about in the passage?

Philadelphia’s PH11-TV invites you to download our new traffic app for your mobile device. The app---131--- the station’s traffic coverage. Use it to get traffic news if you are away from your television and are unable to watch our traffic reports. --132--. Plus, you can program your daily commute into the app to receive personalized alerts ---133--- on your mobile device when any traffic-related event occurs along with your note. Avoid traffic delays by downloading the   PH11-TV traffic app today,---134--- tune in to our live broadcast beginning at 5:00 A.M. and 4:00 P.M. daily.

131...................

Choose the best answer:
Everything is ___________ in space.

Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks
Most Americans eat three  meals (10)...... the day: breakfast, lunch, and dinner. Breakfast begins between 7:00 and 8:00am, lunch between 11:00 am and noon, and dinner between 6:00 and 8:00 pm. On Sundays  "brunch" is  a (11) ..................of breakfast and lunch, typically beginning at 11:00 am. Students often enjoy a "study break" or evening snack around 10:00 or 11:00 pm. Breakfast and lunch tend to be light meals, with only one course. Dinner is the main meal.
For breakfast Americans will eat cereal with milk which are often mixed (12)......... in a bowl, a glass of orange juice, and toasted bread or muffin with jam, butter, or margarine. Another common breakfast meal is scrambled eggs or an omelet with potatoes and breakfast meat (bacon or sausage). People who are on a diet eat just a cup of yogurt. Lunch and dinner are more (13)............ . When eating at a formal dinner, you may be overwhelmed by the number of utensils. How do you (14)..........the difference between a salad fork, a butter fork, and a dessert fork? Most Americans do not know the answer. But knowing which fork or spoon to use first is simple: use the outermost utensils first and the utensils closest to the plate last.
(13)..............................

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions:
Around 365 B.C. in ancient Greece, the great teacher Plato told a story about a place called Atlantis. Plato described Atlantis as a continent in the Atlantic Ocean. He said it had been the home of a powerful people who were destroyed when the continent was swallowed by the sea.
Ever since Plato’s time, people have wondered if the story about Atlantis was really true. Was there ever such a place as Atlantis? During the Middle Ages, many people believed in the legend of Atlantis. Some men even made voyages to find the “lost continent”. Later, most people believed Atlantis to be just a myth. Scientists could find no evidence to show that such a place had ever existed.
Ideas change in time, however, for now some scientists think that Atlantis might have been a real place. A Greek professor has offered a new theory. He says that Atlantis was not a continent in the Atlantic Ocean but an island called Thera in the Aegean Sea. The professor says that 3,500 years ago much of Thera collapsed into the sea when a volcano erupted. He thinks that before the explosion Thera had been the home of a people called Minoans. The Minoans were sea traders who ruled the Aegean Sea from 2,000
B.C. to 1,250 B.C.
Scientist who have come to investigate Thera have found an ancient city buried beneath volcanic ash and stone. It appears that the people who lived in the city had an advanced civilization. They were probably Minoans.
Is Thera the “lost continent” of Atlantis? No one may ever know for certain. Atlantis may remain a riddle without an answer.

Who were the ancient Minoans?

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions:
In the early 1800s, to reach the jump-off point for the West, a family from the East of the United States could either buy a steamboat passage to Missouri for themselves, their wagons and their livestock or, as happened more often, simply pile everything into a wagon, hitch up a team, and begin their overland trek right in their front yard.
Along the macadamized roads and turnpikes east of the Missouri River, travel was comparatively fast, camping easy, and supplies plentiful. Then, in one river town or another, the neophyte emigrants would pause to lay in provisions. For outfitting purposes, the town of Independence had been preeminent ever since 1827, but the rising momentum of pioneer emigration had produced some rival jump-off points. Westport and Fort Leavenworth flourished a few miles upriver. St. Joseph had sprung up 55 miles to the northwest; in fact, emigrants who went to Missouri by riverboat could save four days on the trail by staying on the paddle wheelers to St. Joe before striking overland.
At whatever jump-off point they chose, the emigrants studied guide books and directions, asked questions of others as green as themselves, and made their final decision about outfitting. They had various, sometimes conflicting, options. For example, either pack animals or two -wheel carts or wagons could be used for the overland crossing. A family man usually chose the wagon. It was the costliest and slowest of the three, but it provided space and shelter for children and for a wife who likely as not was pregnant. Everybody knew that a top-heavy covered wagon might blow over in a prairie wind or be overturned by mountain rocks, that it might mire in river mud or sink to its hubs in desert sand, but maybe if those things happened on this trip, they would happen to someone else. Anyway, most pioneers, with their farm background, were used to wagons.

The word “preeminent” in paragraph 2 is closest in meaning to    .

Philadelphia’s PH11-TV invites you to download our new traffic app for your mobile device. The app---131--- the station’s traffic coverage. Use it to get traffic news if you are away from your television and are unable to watch our traffic reports. --132--. Plus, you can program your daily commute into the app to receive personalized alerts ---133--- on your mobile device when any traffic-related event occurs along with your note. Avoid traffic delays by downloading the PH11-TV traffic app today,---134--- tune in to our live broadcast beginning at 5:00 A.M. and 4:00 P.M. daily.

134.....................

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions:
Newspapers can be traced back to 16th century Venice. In 1566, handwritten news sheets - called 'avis' or ‘gazette' – filled with information on wars and politics in Europe were distributed weekly in Venice. Similar news sheets soon started to appear in other European countries. By 1615, Germany and Austria were publishing weeklies. And in 1621, the first news sheets appeared in England.
At first, these news sheets only printed news which came from outside the country in which they were printed. Discussion of local or national issues was avoided. Europe’s governments did not tolerate anything negative being said about them as it could lead to national unrest.
Such censorship slowed the development of newspapers. Nevertheless, a belief in the importance of a 'free press’ slowly began to take hold in Europe. England was among the first countries to escape government control of the press. This occurred during the reign of King Charles I in the 17th century, when, during a period of breakdown in the king's authority, people began to publish what they wanted.
Eventually, free press had the right to criticize government and voice other ideas freely. In the middle of the 18th century, Sweden became the first country to make press freedom a part of its law.
In the 19th century, the newspaper industry was transformed by the invention of the telegraph. The telegraph was a communication system that allowed messages to be sent over long distances in a matter of minutes. It wasn’t long before newspapers became society's primary means of spreading and receiving information. In 1880, the first photographs appeared in newspapers and, by the end of the century, all the basic technical tools for the modern newspaper were in place.
The story of newspapers in the 20th century was one of adaptation to changing consumer and media markets. The invention of radio, TV, and later the Internet, repeatedly drove newspapers to re-invent themselves. Also, during the 20th century, mass-market advertising increased profitability for newspapers. This attracted large, publicly-owned corporations who began buying newspapers from the descendants of company founders.
Over the years, people have periodically predicted the extinction of newspapers. In fact, every time a new media has come into being, dire predictions have been made for existing forms (e.g. television was supposed to have replaced radio, radio was supposed to have replaced newspapers). Yet history has repeatedly shown that new media do not replace existing media. Instead, what happens is that media consumption grows, which creates the necessary space for the new media to become a part of the media landscape.
According to the World Association of Newspapers (WAN), each day more than 1.5 billion people around the world read a newspaper. The WAN has also estimated the total annual worth of the global newspaper industry and put it at just under 180 billion USD. Such statistics suggest the newspaper industry is healthier than at any other time in its history. Indeed, if the industry proves itself as capable of adapting to change as it has done in the past, it is unlikely that newspapers will be disappearing from newsstands anytime soon.

In paragraph 1, we learn that     .

Choose the best answer to complete the passage.

Twenty (1)________ ago, kids in school had never even heard of the internet. Now, I'll bet you can't find a single person in your school who hasn't at least heard of it. In fact, many of us use it on a regular basis and even have access to it from our homes! The 'net' in internet really stands (3) _________ network. A (2) __________  is two or more computers connected together so that information can be (4) __________, or sent from one computer to another. The internet is a vast resource for all types of information. You may enjoy using it to do research for a school project, downloading your favourite songs or communicating with friends and family. Information is accessed through web pages (5) _________ companies, organizations and individuals create and post. It's kind of like a giant bulletin board that the whole world uses! But since anyone can put anything on the internet, you also have to be careful and use your best judgement and a little common sense.

Just because you read something on a piece of paper someone sticks on a bulletin board doesn't mean it's good information, or even correct, for that matter. So you have to be sure that whoever posted the information knows what they're talking about, especially if you're doing (6) ___________! But what if you're just emailing people? You still have to be very careful. If you've never met the person that you're communicating with online, you could be on dangerous ground! You should never give out any personal information to someone you don't know, not even your name! And just like you can't believe the information on every website out there, you can't rely (7) ________ what strangers you 'meet' on the internet tell you either. Just like you could make up things about yourself to tell someone, someone else could do the same to you!

(4)________

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions:
The technology of the North American colonies did not differ strikingly from that of Europe, but in one respect, the colonists enjoyed a great advantage. Especially by comparison with Britain, Americans had a wonderfully plentiful supply of wood.
The first colonists did not, as many people imagine, find an entire continent covered by a climax forest. Even along the Atlantic seaboard, the forest was broken at many points. Nevertheless, all sorts of fine trees abounded, and through the early colonial period, those who pushed westward encountered new forests. By the end of the colonial era, the price of wood had risen slightly in eastern cities, but wood was still extremely abundant.
The availability of wood brought advantages that have seldom been appreciated. Wood was a foundation of the economy. Houses and all manner of buildings were made of wood to a degree unknown in Britain. Secondly, wood was used as fuel for heating and cooking. Thirdly, it was used as the source of important industrial compounds, such as potash, an industrial alkali; charcoal, a component of gunpowder; and tannic acid, used for tanning leather.
The supply of wood conferred advantages but had some negative aspects as well. Iron at that time was produced by heating iron ore with charcoal. Because Britain was so stripped of trees, she was unable to exploit her rich iron mines. But the American colonies had both iron ore and wood; iron production was encouraged and became successful. However, when Britain developed coke smelting, the Colonies did not follow suit because they had plenty of wood and besides, charcoal iron was stronger than coke iron. Coke smelting led to technologic innovations and was linked to the emergence of the Industrial Revolution. In the early nineteenth century, the former colonies lagged behind Britain in industrial development because their supply of wood led them to cling to charcoal iron.

According to the passage, why was the use of coke smelting advantageous?

Read the following passage and mark the letter A,B,C or D on your answer sheet to indicate the correct word for each of the blanks
                           Swim, Cycle, and Run
For athletes who like to (1).............. a variety of endurance sports in a single race, triathlons are a great fit. These competitions (2)......... running, cycling, and swimming into one race, and they take place one after the other in a single continuous timed race. Athletes who participate must be in top physical form and have the skills needed to complete each portion of the race.
 Training for triathlons means the athletes have to become proficient not only in the sport required in each stage, but they also have to become familiar with what (3).............  from one to the other does to their bodies. Many inexperienced triathletes are surprised to (4)............... hard it can be to switch from swimming to cycling to running, and how tired their muscles can suddenly feel. One thing all triathletes can agree (5).................is that no one ever told them that running a triathlon would be easy.

(3)..............................

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions:
It can be shown in facts and figures that cycling is the cheapest, most convenient, and most environmentally desirable term of transport in towns, but such cold calculations do not mean much on a frosty winter morning. The real appeal of cycling is that it is so enjoyable. It has none of the difficulties and tensions of other ways of traveling so you are more cheerful after a ride, even though the rush hour.
The first thing a non-cyclist says to you is: "But isn't it terribly dangerous?" It would be foolish to deny the danger of sharing the road with motor vehicles and it must be admitted that there are an alarming number of accidents involving cyclists. However, although police records indicate that the car driver is often to blame, the answer lies with the cyclist. It is possible to ride in such a way as to reduce risks to a minimum.
If you decide to join the thousands in Britain who are now returning to cycling as a cheap, satisfying form of transport your first problem will be trying to decide what bike to buy. Here are three simple rules for buying a bike:
Always buy the best you can afford. Of course there has to be a meeting point between what you would really like and economic reality, but aim as high as you can and you will get the benefit not only when you ride but also if you want to sell. Well-made bikes keep the value very well. And don't forget to include in your calculations the fact that you'll begin saving money on fares and petrol the minute you leave the shop.
Get the best frame, the main structure of the bicycle, for your money as you can. Cheap brakes, wheels or gears can easily be replaced by more expensive ones, but the frame sets the    upper limit on any transformation. You should allow for the possibility our cycling ambitions will grow with practice. When you begin, the four miles to work may the most you ever dream of, but after a few months a Sunday ride into the country begins to look more and more desirable. The best thing is to buy a bike just a little bit better than you think you'll need, and then grow into it. Otherwise, try to get a model that can be improved.
The fit is vital. Handlebars and seat height can be adjusted but you must get the right sized frame. On the whole it is best to get the largest size you can manage. Frame sizes are measured in inches and the usual adult range is from 21 inches to 25 inches, though extreme sizes outside those measurements can be found. Some people say if you take four inches off from your inside leg measurement you will end up with the right size of bike. The basic principle though is that you should be able to stand with legs either side of the crossbar (the bar that goes from the handlebars to the seat) with both feet comfortably flat on the ground.

After you have been cycling for a few months the author suggests that you will

Choose the best answer:
How is your holiday in Dalat city? __________.

First, the site is not as ---143--- as it should be. We recommend updating its appearance and adding information that meets the demands of today’s tourists. Note also that your organization’s logo is not used consistently ---144--- your Web site.
You should also consider supplementing the imagery used to promote the city. 145---. We therefore recommend uploading some professionally made videos featuring the various attractions Brookside has to offer. ---146---, we suggest adding a page to the Web site that allows residents and visitors to upload their own photos and videos of city attractions.

144....................

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks.
The (1)........... of the earth is caused by exhaust gas from automobile engines, factories and power  stations. Carbon dioxide goes up into the atmosphere, and it forms a kind of factories that keeps or rather allows the sunshine in but stop the earth’s heart (2).........  getting out. It works like a greenhouse, that’s why we call it the Greenhouse Effect.
Because of this effect, the earth is getting warmer all the time. This (3).........    in temperature will  cause big changes to the world's climate. The sea level will rise, the ice covering the poles will melt and cause the sea level to rise.
The second problem is the (4)......... of the ozone layer. Certain chemicals float up to the sky and react with the ozone layer, and they make holes in it. Because of these holes the ultraviolet rays can shine directly to the earth. Many people are now starting to suffer from skin cancer.
The sulfurous smoke from factories and power stations mixes with rain clouds and gets blown by the wind and then falls as acid rain (5)......... destroys lakes and forests.
These three problems threaten our environment at the end of the twentieth century. Unless we do something about them quickly, we, human race, may disappear from the face of the earth.

(1)...........................

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions:
If the salinity of ocean waters is analyzed, it is found to vary only slightly from place to place. Nevertheless, some of these small changes are important. There are three basic processes that cause a change in oceanic salinity. One of these is the subtraction of water from the ocean by means of evaporation-conversion of liquid water to water vapor. In this manner, the salinity is increased, since the salt behind. If this is carried to the extreme, of course, white crystals of salt would be left behind.
The opposite of evaporation is precipitation, such as rain, by which water is added to the ocean. Here the ocean is being diluted so that the salinity is decreased. This may occur in areas of high rainfall or in coastal regions where rivers flow into the ocean. Thus salinity may be increased by the subtraction of water by evaporation, or decreased by the addition of fresh water by precipitation or runoff.
Normally, in tropical regions where the sun is very strong, the ocean salinity is somewhat higher than it is in other parts of the world where there is not as much evaporation. Similarly, in coastal regions where rivers dilute the sea, salinity is somewhat lower than in other oceanic areas.
A third process by which salinity may be altered is associated with the formation and melting of sea ice. When seawater is frozen, the dissolved materials are left behind. In this manner, seawater directly beneath freshly formed sea ice has a higher salinity than it did before the ice appeared. Of course, when this ice melts, it will tend to decrease the salinity of the surrounding water.
In the Weddell Sea, off Antarctica, the densest water in the ocean is formed as a result of this freezing process, which increases the salinity of cold water. This heavy water sinks and is found in the deeper portions of the oceans of the world.

According to the passage, the ocean generally has more salt in .

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions:
Humans have struggled against weeds since the beginnings of agriculture. Marring our gardens is one of the milder effects of weeds - any plants that thrive where they are unwanted. They clog
waterways, destroy wildlife habitats, and impede fanning. Their spread eliminates grazing areas and accounts for one-third of all crop loss. They compete for sunlight, nutrients, and water with useful plants. The global need for weed control had been answered mainly by the chemical industry. Its herbicides are effective and sometimes necessary, but some pose serious problems, particularly if misused.
Toxic compounds threaten animal and public health when they accumulate in food plants, groundwater, and drinking water. They also harm workers who apply them.
In recent years, the chemical industry has introduced several herbicides that are more ecologically sound. Yet new chemicals alone cannot solve the world's weed problems. Hence, scientists are exploring the innate weed-killing powers of living organisms, primarily insects and microorganisms.
The biological agents now in use are environmentally benign and are harmless to humans. They can be chosen for their ability to attack selected targets and leave crops and other plants untouched. In contrast, some of the most effective chemicals kill virtually all the plants they come in contact with, sparing only those that are naturally resistant or have been genetically modified for resistance. Furthermore, a number of biological agents can be administered only once, after which no added applications are needed. Chemicals typically must be used several times per growing season.

Which of the following best describes the organization of the passage?