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

新任企业工会 * * 、副 * * ，应在（）内参加上级工会举办的上岗资格或业务培训。

A.一年

B.两年

C.三年

D.四年