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 ad

The great advance in rocket theory 40 years ago showed that liquid-fuel rockets 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 about as powerful as liquid fuels have made their appearance, and it is a favorite layman’s question to inquire which one is "better".(16)The question is meaningless; one might as weft" ask whether a gasoline or a diesel engine is "better". It all depends on the purpose.(18)/(19)A liquid fuel rocket is complicated, but has the advantage that it can be controlled beautifully. The burning of the rocket engine can be stopped completely; it can be reignited when desired. In addition, 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 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 very short notice. A liquid-fuel rocket has to be fueled first and cannot be held in readiness for very long after it has been fueled. However, once a solid-fuel rocket has been ignited, it will keep burning. It cannot be stopped and reignited whenever desired(it could conceivably be stopped and reignited after a pre-calculated time of burning has elapsed)and its thrust cannot be varied. Because a solid-fuel rocket can be kept ready for a long time, most military missiles employ solid fuels,(18)but manned spaceflight needs the fine adjustments that can only be provided by liquid fuels.(19)It may be added that a liquid-fuel rocket is an expensive device; a large solid-fuel rocket is, by comparison, cheap.(17)/(19)But the solid fuel, pound per pound, costs about 10 times as much as the liquid fuel.(19)So you have, on the one hand, an expensive rocket with a cheap fuel and on the other hand a comparatively cheap rocket with an expensive fuel.
Questions 16 to 19 are based on the passage you have just heard.
16. What conclusion can we draw from the passage?
17. Why are the solid-fuel rockets expensive to operate?
18. What’s the most important consideration for the rocket that used for manned spaceflight?
19. Which of the following statements is NOT the characteristic of liquid-fuel rockets?