单选题 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.

【正确答案】 D

【答案解析】[解析] 事实细节题。从文章第二段第四句，Therefore，by producing a perfect copy of the noise and delaying it by half a wave cycle， we can kill the unwanted noise可知，降低噪音的原理是产生一个完全相同的声波，然后让它延迟半个周期，我们就可以消除令人讨厌的噪声了。所以正确的答案为D。