971.Although the causes of cancer ________,we do not yet have any practical way to prevent it.

A.are being uncovered

B.have been uncovering

C.are uncovering

D.have uncovered

972.Oh, God! What did you do to my report, Tom?
“Terribly sorry. My mind _____ and I will type it again for you.

A.has wandered

B.had wandered

C.is wandering

D.was wandering

971.答案是A;

由后半句的not yet得知uncover这一动作仍在进行中，且为被动。

972.答案是D;

考查动词的时态。根据上文时态的暗示，此处指当时大脑处于游离状态，故选D。