单项选择题

NASA’s Mars detectot，Opportunity，succeeded in finding signs that water once existed on the planet．It is now believed that the planet（）life．

A.should once support
B.should once have supported
C.could once support
D.could once have supported

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单项选择题
why（）we eat out when we can easily cook ourselves at home？

A.must
B.shall
C.can
D.may
单项选择题
You（）not leave your post．or you will be punished．

A.may
B.shall
C.will
D.need
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