问题补充:
over and over again, but he still couldn’t understand.
A. Having explained
B. Having been explained
C. Explaining
D. It had been explained
答案:
A. Having explained
B. Having been explained
C. Explaining
(答案→)D. It had been explained
解析:本题考查时态。本题根据but he still couldn’t understand为过去时,故从句用过去完成时。句意:它解释了一遍又一遍,但他仍无法理解。