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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.
Subsonic flight does not produce a sonic boom, because it ______.
选项 A、does not disturb existing pressures
B、stays behind its disturbance
C、does not produce great enough changes in pressure
D、stays ahead of its disturbance
答案D
解析 文章第二段第二句指出，亚音速飞行的飞机产生轻微的压力变化，但是由于这种干扰以音速行驶，所以亚音速干扰比飞机飞行快，并保持在飞机的前面。
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