A．They built a much lighter rocket.
B．They built a much faster rocket.
C．They loaded the rocket with enough fuel.
D．They built a multistage rocket.

A.They
B.
B．They
C.
C．They
D.
D．They

【参考答案】

D
解析：在制造火箭推进器的过程中，科学家们意识到it would be difficult for one h......

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