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  3. To fight a noise by making more noise sounds strange, but that’ s exactly what motor engineers are doing in Germany and some oth

To fight a noise by making more noise sounds strange, but that’ s exactly what motor engineers are doing in Germany and some oth

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问题     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 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 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.
The noise killing system is not yet popular mainly because______.
选项 A、it is still being tried out
B、drivers still have their doubts
C、it increases the cost of car production
D、carmakers are not sure if it is necessary
答案C
解析 推理判断题。从文章最后一段最后一句，But the carmakers haven’t decided if they willcarry it into production because it would add several hundred dollars to the cost of their cars．可知，因为费用太高，所以还未确定是否投入生产。
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