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  3. Under proper conditions, sound waves will be reflected from a hillside or other such obstruction. Sound travels at the rate of a

Under proper conditions, sound waves will be reflected from a hillside or other such obstruction. Sound travels at the rate of a

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问题     Under proper conditions, sound waves will be reflected from a hillside or other such obstruction. Sound travels at the rate of about one-fifth of a mile per second. If the hill is eleven hundred feet away, it takes two seconds for the sound to travel to the hill and back. Thus, by timing the intervals between a sound and its reflection (the echo), you can estimate the distance to an obstruction.
    During World War II the British used a practical application of this principle to detect German planes on their way to bomb London long before the enemy was near the target. They used radio waves instead of sound waves, since radio waves can penetrate fog and clouds. The outnumbered Royal Air Force (RAF) always seemed to the puzzled Germans to be lying in wait at the right time and never to be surprised. It was radio echoes more than anything else that won the Battle of Britain.
    Since the radio waves were used to tell the direction in which to send the RAF planes and the distance to send them (their range of flight, in other words) , the device was called radio directing and ranging, and from the initials the word radar was coined.
Sound waves reflected from a hill can be used to estimate the______.
选项 A、height of the hill
B、speed of sound
C、distance to the hill
D、intensity of sound
答案C
解析 文章第一段最后一句说通过对声音和回音之间的间隔进行计时，可以算出与障碍物之间的距离，故选C。
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