[单项选择]Sound moves from its source to the ear by wave like fluctuations in air pressure, something like the peaks and troughs or the lowest point of ocean waves. One way to keep from hearing sound is to use ear plugs. Another way is to cancel out the sound with anti-sound.
Using a noise maker controlled by a microprocessor, engineers have produced sound waves that are half a wavelength out of phase with those of the noise to be quieted--each peak is matched to a trough, and vice versa. Once the researchers have recorded the offending sound, a microprocessor calculates the amplitude and wavelength of sound that will cancel out the highest and lowest points of the noise. It then produces an electric current that is amplified and fed to a loudspeaker, which produces anti-sound and wipes out the noise. If the anti-sound goes out of synchronization, a microphone picks up the leftover sound and sends it back to the microprocessor, which changes the phase of the anti-sound just enough to cause
A. the rising and falling of water in the ocean
B. the crests and valleys of mountain ranges
C. a flag waving in the air
D. a machine for measuring air pressure

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