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  3. Among the question of design, construction, and operation remaining to be solved before commercial airliners travel faster than

Among the question of design, construction, and operation remaining to be solved before commercial airliners travel faster than

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问题     Among the question of design, construction, and operation remaining to be solved before commercial airliners travel faster than sound is the difficult problem of sonic boom (声震) , the explosive sound generated when an object moves through the air at supersonic speed(超音速).
    A sound is a pressure disturbance: a sonic boom, like other explosive sounds, is the result of an abrupt change in pressure. An airplane in subsonic flight produces weak changed in pressure, but because these disturbances travel at the speed of sound, they move faster than the airplane and stay in front of it. In effect, these disturbances warn the air to get out of the way, and the an does just jet, however, gets ahead of its own pressure disturbances. Consequently a wave of suddenly com pressed air--a shock wave--builds up and is thrown off like the wave ;hat spreads from the bow of a speedboat.
    Can sonic booms be eliminated? The answer is a categorical negative--they are an innerent(内在的) part of supersonic flight--but the sonic boom probably can be reduced; government and private research is now under way to improve understanding of the sonic boom. If and when the booms can be reduced to tolerable levels, commercial supersonic transports can come rata service.
Research is under way to ______.
选项 A、discover what causes the sonic boom
B、find a means of eliminating the sonic boom
C、produce the sonic boom at low altitudes
D、reduce the sonic boom to tolerable levels
答案D
解析 文章第三段指出，现在正着力于更多地了解声震现象，如果能使声震降到一定程度，商务超音速运输就可以运行服务了。
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