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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.
It can be inferred from the passage that______.
选项 A、light waves could be used in a device similar to radar
B、radar was a practical application of a well-known principle
C、radar gready increased the effectiveness of the Royal Air Force
D、sound waves are reflected from a hillside under all conditions
答案C
解析 雷达大大提高皇家空军的效率。第二段一句话“During World warⅡ…was near the target”和第三、四句话“The outnumbered Royal Air Force…won the Battle of Briton”都是围绕这个中心展开的。A项中的光波是错误的。B项中雷达是一个众所周知原理的应用。这个原理不是众所周知或常识性的，而是比较专业的。D选项中出现任何表示“绝对”意义的词汇，比如not at all，by no means，under no circumstance
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