He was surprised at____about it

题目

He was surprised at____about it
A.having asked B.having been asked
C.being asked D.asking
答案应该选B,为什么不能选C?请说明原因,

答案

因为他被问到这个是在他感到吃惊之前,原句中主句是过去时,所以从句由过去时变为过去完成时.