问题
选择题
Although there is no life on Mars now , some scientists think that there some form of life a long time ago .
A．must be
B．may have been
C．can have been
D．could be
答案
答案：B
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Although there is no life on Mars now , some scientists think that there some form of life a long time ago .
A．must be
B．may have been
C．can have been
D．could be
答案：B