2. 外语
  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 interval between a sound and its reflection (the echo), you can estimate the distance to an obstruction.
    During World War I1 the British used a practical application of this principle to detect German planes on their way to bomb London 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 (袭击). 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 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______.
选项 A、height of the hill
B、speed of sound
C、distance of the hill
D、intensity of sound
答案C
解析 从第一段我们知道，声波可以从山坡反射回来，声音运行的速度为每秒约五分之一英里，如果一座山的距离是1100英尺，那么声音到达那里再返回需要两秒钟，从这里我们知道正确答案为C；A．山的高度；B．音速；D，声音的强度，均与原文不符。
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