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 fueling.
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单项选择题
Solid fuel rockets are expensive to operate because of their ______.
A．size
B．fuel
C．construction
D．complicated engines
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