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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 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.
Radar enabled the English ______.
选项 A、detect German planes on their way to London
B、direct the outnumbered RAF planes effectively
C、confuse German bomber pilots
D、both A and B
答案A
解析 从文章第二段第一句我们知道，在二战期间，英国人运用这一原理来探测敌人接近目标之前到伦敦的距离，这里所说的实际运用这一原理就是指雷达，所以正确答案为A；B．有效地指挥数量上占优越的皇家飞机，这也不对的，雷达可以探测敌机距离，但是人运用它来指挥飞机；C．迷惑德国轰炸机飞行员，这也不对，雷达没有这个功能；D．把A和B两项放在一起不符原文。
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