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 land them on the moon. We have now become space 【C3】______ .
   Impressive and complex as it may appear, the rocket, which was 【C4】______ in China over 800 years ago, is a relatively simple device. Fuel that is 【C5】______ in the rocket engine changes 【C6】______ gas. The hot and rapidly 【C7】______ gas must escape, but it can do so only through an opening that 【C8】______ backward. As the gas is 【C9】______ with great force, it 【C10】______ the rocket in the 【C11】______ direction. Like the kick of a gun 【C12】______ it is fired, it 【C13】______ the laws of nature 【C14】______ by Sir Isaac Newton when he discovered that" 【C15】______ every action, there is an equal and opposite reaction."
【C1】______

选项 A、shown
B、been
C、appeared
D、proved

答案D

解析  因其后接形容词powerful作表语故应填系词。shown不是系词;been和appeared虽可作系词但与句意不符;而proved可作系词,符合句意。
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