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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 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 by 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 rage 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 to______.
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D
解析 从第二段第一句话“During World’WarⅡ…before the enemy was near the target”可以得出A正确。可是该题容易漏选。考生常常选出A就看下一题，当看到选项D的时候，就应该回头看看B是否能选。B参见第二段第三句话：The outnumbered Royal Air Force(RAF)always seemed…never to be surprised数量上占优势的皇家空军飞机似乎总是镇定自若地在恰当的时间等待那群迷惑不解的德军的到来。从结构讲，这句话是该段第一
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