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 with 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 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, which not only reduces the weight of a car, but also makes the motor burn less oil and work better.

　　Some engineers believe that the noise-killing system will be used in most cars in l996. But the carmakers haven't decided if they will put it into production because it would add several hundred dollars to the cost of their cars.

　　1. Which of the following gives a general idea of how the noise-killing system works?

　　A.By producing pressure waves going in the opposite direction.

　　B.By mising high frequency sound waves with low frequency sound waves.

　　C.By making copies of the unwanted sound waves and letting them out a little later.

　　D.By mixing new sound waves with the noise and sending them out together.

　　2. Besides its main function (功能) the noise-killing system can______.

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