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

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热门试题

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

单项选择题

Alan Johnson decided to leave New York because it is__________.

A．dirty
B．crowded
C．indifferent
D．unpleasant