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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.
Sound waves reflected from a hill can be used to estimate the______.
选项 A、height of the hill
B、speed of sound
C、distance to the hill
D、intensity of sound
答案C
解析 A．山的高度。用这种方法测不出山的高度，文中也没有提及“高度”这个概念。B声速。声速是已知的，也不需要用这种方式来测定。C．离山的距离。第一段最后一句话：Thus，by timing the interval…the distance to and obstruction．通过给声波和回声间隔计时，可以估计和障碍物的距离。这里的“障碍物”在该段第一句中曾提及，“under proper conditions or other such obstruction”即山或山坡。D．声音的密度。这个概念在文中没有
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