Although there is no life on Mars now some scientists think that there some form of life

问题补充：

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：

A. must be

(答案→)B. may have been

C. can have been

D. could be

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