According to the passage, what does a constricted and harsh voice indicate?

The word "they" in the last paragraph refers to ...............

The word "all-inclusive" in the passage mostly means ................

we forget that they must have been gloomy and murky places at night. It is mentioned in the passage that before the electric lamp was invented .............

Medical students have to follow a course lasting for ............

If you have a desire to travel, when does the writer suggest that you do it?

What is the most important purpose of the ozone layer?

What can be inferred about machines that are not calculators or computers?

As a result of pollution, Lake Erie, on the borders of the USA and Canada, is now without any living things. Pollutions in water are not simply a matter of “poisons” killing large numbers of fish overnight. Very often the effects of pollution are not noticed for many months or years because the first organisms to be affected are either plants or plankton. But these organisms are the food of fish and birds and other creatures. When this food disappears, the fish and birds die in this way a whole food chain can be wiped out, and it is not until dead fish and water birds are seen at the river’s edge or on the sea shore that people realize what is happening. Where do the substances which pollute the water come from? There are two main sources – sewage and industrial waste. As more detergent is used in the home, so more of it is finally put into our rivers, lakes and seas. Detergents harm water birds dissolving the natural substance which keep their feather waterproof. Sewage itself, if it is not properly treated, makes the water dirty and prevents all forms of life in rivers and the sea from receiving the oxygen they need. Industrial waste is even more harmful since there are many high poisonous things in it, such as copper and lead. So, if we want to stop this pollution, the answer is simple, sewage and industrial waste must be made clean before flowing into the water. It may already be too late to save some rivers and lakes, but others can still be saved if the correct action is taken at once. 

According to the passage, the way to stop water pollution is ...........

Pollution in water is noticed ............. 

The word “it” in paragraph 1 refers to ..............