问题
选择题
______ 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
答案：D
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。