单项选择题

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 purified.
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单项选择题
Whether there is sufficient nitrogen on Mars is important because it_________.
A．can raise the temperature of Mars
B．can protect living beings from harmful rays
C．can change the atmosphere of Mars
D．is an important part of the air people breathe
单项选择题
Scientists are interested in Mars because_______________.
A．people might have to migrate there someday
B．people could live there naturally
C．life has ever existed there
D．its atmosphere is quite different
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