[简答题]Unwanted sound, or noise, such as that produced by airplanes, traffic, or industrial machinery, is considered a form of pollution.

[单项选择]To fight a noise by making more noise sounds strange, but that’s exactly what motor engineers are doing in Germany and some other countries.
Carmaker’s research and development laboratories have already proved that mixing more noise with the help of loudspeakers can reduce the unwanted noise. Physicists have known about the technique for a long time. Sound is made up of pressure waves in the air. If two sound waves of the same frequency (频率) mix so that the highest point of one wave happens at the same time as the lowest point of the other wave, the result is no sound. Therefore, by producing a perfect copy of the noise and delaying it by half a wave cycle (周期), we can kill the unwanted noise. Using this technique many carmakers are racing to develop noise killing systems (系统) both inside and outside the cars.
Another good thing about the use of noise killing systems is that it saves the need for a silencer; this not only reduces the weight of a car, but also makes the mo
A. By producing pressure waves going in the opposite direction.
B. By mixing high frequency sound waves with low frequency sound waves.
C. By mixing new sound waves with the noise and sending them out together.
D. By making copies of the unwanted sound waves and letting them out a little later.

[单项选择]Unloved and unwanted youngsters may be tempted to run away from home to escape their problems, ______ bigger ones in cities plagued with crime, drugs, and immorality.
A. have only found
B. only finding
C. only found
D. only to find

[单项选择]The chocolate were not produced
A. for more than ten years.
B. for less than ten years.
C. during the World War II.

[填空题]Who will notice the noise most

[单项选择]A. In areas with few weeds and unwanted plants.
B. In areas with a severe shortage of water.
C. In areas lacking in chemical fertilizer.
D. In areas dependent on imported food.

[单项选择]As the silent film（） sound, so did the sound film（）color.
A. cried out for...cried out for
B. cry out for...cry out for
C. had cried out for...cried out for
D. had cried out for...cry out for

[填空题]Notice a sound.

[单项选择]Where did the strange noise come from
A. She turned on the light.
B. She closed the window.
C. She closed the door.