摘要：B Under proper conditions, sound waves will be reflected from a hillside or other such obstruction. Sound travels at the speed 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 measuring the time between a sound and its reflection the echo, you can calculate the distance to an obstruction. During World War II the British made a practical use of this role to discover German planes on their way to bomb London long before the enemy was near the object. They used radio waves instead of sound waves, since radio waves can pass 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 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 range of flight, in other words) ,the device was called radio directing and ranging, and from the first letters the word radar was formed. 1. Sound waves reflected from a hill can be used to calculate the . A. speed of sound B. time between a sound and its echo C. height of the hill D. distance to the hill 2. The British used radio waves because they . A. were more exact than sound waves B. could not be discovered C. could pass through fog and clouds D. were easier to use than sound waves 3. Practical use of radio waves brought about . A. new machines for planes B. a radio directing and ranging device C. new radio sets for BAF bombers D. a discovering device 4. Radar made the British be able to . A. discover German planes on their way to London B. direct the outnumbered RAF planes effectively C. kill German bomber pilots D. both A and B 5. The author of this passage mainly wanted to tell . A. exactly how radar worked B. why the British used radio waves in their device C. how radar came into being D. how radar helped the British win the Battle of Britain

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