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

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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【C1】______powerful enough to shoot astronauts【C2】______the earth’s gravitational pull and【C3】______them on the moon. We have now become【C4】______in space. Yuri Gagarin from Soviet Union was the first human to visit space on April 12, 1961. In the 108 minutes’ journey, he covered 40, 000 kilometers. China’s first astronaut into space is Yang Liwei. Impressive and complex【C5】______it may appear, the rocket, which was【C6】______in China over 800 years【C7】______, is a relatively simple device. Fuel that is【C8】______in the rocket engine changes【C9】______gas. The hot and rapidly【C10】______gas must escape, but it can do so only【C11】______an opening that【C12】______backward. As the gas is【C13】______with great force, it【C14】______the rocket in the【C15】______direction. Like the kick of a gun【C16】______it is fired, it【C17】______the laws of nature【C18】______by Sir Isaac Newton when he discovered that "【C19】______every action, there is【C20】______equal and opposite reaction. " Newton’s theories have contributed a great deal to the science of aeronautics.
【C7】

选项 A、in advance
B、before
C、earlier
D、ago

答案D

解析 该句为which引导的定语从句,意为“中国发明火箭是在距今800多年前。”句中谓语是一般过去时,故应选ago。ago指从现在算起;be-fore是从过去某个时刻算起,且多用于现在完成时态中;earlier是early的比较级,更早的;in ad—vance意为“预先”,都不符合句意。
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