-----How long do you think it will be man can finally solve the problem of cancer?------We

问题补充：

-----How long do you think it will be man can finally solve the problem of cancer?

------Well．it depends．

A. when

B. until

C. after

D. before

答案：

A. when

B. until

C. after

(答案→)D. before

解析：考查连词。本题考查it will … before …还有多久才能……。句意：——你认为人类还要多久才能解决癌症问题？——嗯，那还得看情况。故D正确。