The great advance【C1】______rocket theory 40 years ago showed that liquid-fue

游客2025-01-22 25

问题 The great advance【C1】______rocket theory 40 years ago showed that liquid-fuel rockets were far【C2】______in every respect to the skyrocket with its weak solid fuel, the only kind of rocket then known.【C3】______, during the last decade, large solid-fuel rockets with【C4】______fuels about as powerful as liquid fuels have【C5】______their appearance, and it is a favorite layperson’s question to【C6】______which one is "better". The question is meaningless.【C7】______might as well ask whether a gasoline or a diesel engine is "better". It all【C8】______on the purpose. A liquid-fuel rocket is complicated,【C9】______has the advantage that it can be controlled beautifully. The burning of the rocket engine can be stopped completely; it can be reignited when【C10】______.【C11】______, the thrust can be made to vary by adjusting the speed of the fuel pumps. A solid-fuel rocket, on the other hand, is rather【C12】______in construction,【C13】______hard to build when a really【C14】______size is desired. But once you have a solid-fuel rocket, it is ready for action【C15】______very short notice. A liquid-fuel rocket has to be fueled first and cannot be【C16】______readiness for very long after it has been fueled.【C17】______, once a solid-fuel rocket has been ignited, it will keep burning. It cannot be stopped and reignited【C18】______desired(it could conceivably be stopped and reignited after a pre-calculated time of burning has e-lapsed. and its thrust cannot be varied. Because a solid-fuel rocket can be kept ready for a long time, most military missiles【C19】______solid fuels, but human-piloted spaceflight needs the fine adjustments that can【C20】______be provided by liquid fuels. [br] 【C20】

选项 A、not
B、all
C、enough
D、only

答案 D

解析前文已经提到液体燃料火箭有操作起来得心应手的优点，所以此处所说的可以有细微调节功能的人类驾驶的航天飞机必定是液体燃料提供助力。

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