-----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
网友回答
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。