听力原文：The great advance in rocket theory 40 years ago showed that liquid-fuel rackets w

听力原文： The great advance in rocket theory 40 years ago showed that liquid-fuel rackets were far superior in every respect to the skyrocket with its weak solid fuel, the only kind of rocket then known. However, during the last decade, large, solid-fuel rockets with solid fuels a bout as powerful as powerful fuels have made their appearance, and it is a favorite layman's question to inquire which one is "better". The question is meaningless; one might as well as ask whether a gasoline or a diesel engine is "better", it all depends on the purpose. A liquid-fuel rocket is complicated, but has the advantage that it can be controlled beautifully. The bunting of the rocket engine can be stopped completely; it can be reignited when desired. A solid-fuel rocket, tin the other hand, is rather simple in construction, though hard to build when a really large size is desired. But once you have a solid-fuel rocket, it is ready for action at a very short notice. A liquid fuel rocket has to be fueled flint and Carl not be held in readiness in very long slier it has been fueled. However, once a solid-fuel rocket has been ignited, it will keep burning. It can't be stopped and reignited whenever desired. Because a solid fuel rocket can be kept ready for a long time, most missiles employ solid fuels, but manned space flight needs the line adjustments that can only be pro vided by liquid-fuel rocket. It may be added that a liquid-fuel rocket is an expensive device; a large solid-fund rocket is, by comparison cheap.

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A．Neither of the two types of rockets is very economical.

B．The liquid-fuel rocket is best.

C．Each type of rocket has certain advantages.

D．The solid-fuel rocket is best.