单项选择题

Which of the following statements is not characteristic of liquid-fuel rockets

A．The fuel is cheap.
B．They are cheap to build.
C．They can be stopped and re-ignited.
D．They must be used soon after fuelin
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单项选择题
The most important consideration for manned space flight is that the rocket be ______.
A．inexpensive to construct
B．capable of lifting heavy spacecraft into orbit
C．easily controlled
D．inexpensive to operate
单项选择题
The author tells us that
A．whether a liquid-fuel or a solid-fuel rocket is better depends on the purpose
B．neither type is superior
C．forty years ago, large solid-fuel rockets with solid fuels as powerful as liquid fuels were made
D．the thrust can be made to vary by adjusting the direction of the pump
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