The writer uses the phrase “On the other hand” in the last paragraph to .......

Sound moves form its source to the ear by wavelike fluctuations in air pressure, something like the crests and troughs of ocean waves. Once way to keep from hearing sound is to use ear plugs. Another way is to cancel out the sound with anti-sound. Using a noisemarker controlled by a microprocessor, engineers have produced sound waves that are half a wavelength out of phase with those of the noise to be quieted-each crest is matched to a trough, and vice versa. Once the researchers have recorded the offending sound, a microprocessor calculates the amplitude and wavelength of sound that will cancel out the crests and troughs of noise. It then produces an electric current that is amplified and fed to a loudspeaker, which produces antisound and wipes out the noise. If the anti-sound goes out of synchronization, a microphone picks up the leftover sound and sends it back to the microprocessor, which changes the phase of the anti- sound just enough to cause complete silence. The research team has concentrated on eliminating low-frequency noise from ship engines, which causes fatigue that can impair the efficiency and alertness of the crew, and may mask the warning sounds of alarm and fog signals. 

What is the main purpose of the passage?

The author indicates that the ruby-throated hummingbird is found _______.

Which of the following properties is NOT mentioned as protected by copyright?

The word cherished in paragraph 1 is closest in meaning to_______________

How do theorists believe a person can remember more information in a short time?

The author uses the word “But” in paragraph 4 to ______.

Which of the following terms does the author define in the first paragraph?

It can be inferred from the passage that tsunamis ________ .

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

How did Juliette Davidson benefit from the experience of her first job?