单项选择题

The author feels that a comparison of liquid and solid-fuel rockets shows that ______.

A．neither type is very economical
B．the liquid-fuel rocket is best
C．each type has certain advantages
D．the solid-fuel rocket is hest
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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
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
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