单选题
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”．