over and over again but he still couldn’t understand.A. Having explainedB. Having been e

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。