题目

英语阅读理解 急
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 go through 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 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 i

提问时间：2021-01-23

答案

6.B) distance to the hill
7.A) a radio detecting and ranging device
8.C) detect German planes on their way to London
9.D) Because they could go through fog and clouds.
10.A) Radar was a practical application of a well-known principle.