题目内容：
根据下列文章，回答题。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 thesame 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 thistechnique 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 motor burn less fuel and work better.
Some engineers believe that the noise killing system will be used in most cars within a few years. But the carmakers haven' t decided if they will carry it into production because it would add several hundred dollars to the cost of their cars.
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 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.
参考答案：
答案解析：