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.
Practical application of this principle resulted in ______.
选项 A、new electronic instruments for planes
B、new radio sets for RAF bombers
C、an electronic detecting device
D、a radio directing and ranging device
答案D
解析 从文章的最后一段我们知道，因为使用电波告诉皇家空军飞行方向和飞机距离，所以这种装置就被称作无线电探测器，从它的首字母缩略就创造了radar(雷达)一词，所以正确答案为D；A．新的飞机电子装置；B新的皇家轰炸机无线电装置；C．电子探测器，这三个选项均不十分准确。
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