【单选题】What is one of the things that must be done before man can live on Mars
A.Its surface must be studied.
B.Its temperature must be raised.
C.Big spaceships must be built.
D.Its air must be purifie
【单选题】What is one of the things that must be done before man can live on Mars
A.Its surface must be studied.
B.Its temperature must be raised.
C.Big spaceships must be built.
D.Its air must be purifie
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