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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.
The author of this passage probably intended to explain ,______.
选项 A、exactly how radar works
B、why the British used radio waves in their device
C、how radar(word and device)came to be
D、how radar helped the British win the Battle of Britain
答案C
解析 该题目是一个主旨题。文章内容是阐述了雷达是一个利用声波原理，使用无线电波制造出来，并在二战中发挥巨大作用的装置。而且，“雷达"是一个制造出来的词，是“无线电探测器"的首字母缩写，所以C项“雷达”如何产生为正确答案。
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