The rocket engine,with its steady roar like that of a waterfall or a thunderstorm,is an impressive symbol of the new space age.R

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问题 The rocket engine,with its steady roar like that of a waterfall or a thunderstorm,is an impressive symbol of the new space age.Rocket engines have【B1】_____powerful enough to shoot astronauts【B2】_____the earth’s gravitational pull and【B3】_____them on the moon.We have now become【B4】_____space.
    Impressive and complex【B5】_____it may appear。the rocket。which was【B6】_____in China over 800 years【B7】_____,is a relatively simple device.Fuel that is【B8】_____in the rocket engine changes【B9】_____gas.The hot and rapidly【B10】_____gas must escape,but it can do so only【B11】_____an opening that【B12】_____backward.As the gas is【B13】_____with great  force,it 【B14】_____  the rocket in the【B15】_____direction.Like the kick of a gun 【B16】_____ it is fired,it 【B17】_____the laws of nature  【B18】_____ by Sir Isaac Newton when he discovered that“ 【B19】_____every action.There is【B20】_____equal and opposite reaction”.
【B20】

选项 A、n0  
B、an  
C、another
D、the

答案B

解析 根据上下文,只能用不定冠词an。
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