______ over and over again, but he still couldn’t understand.
A．Having explainedB．Having been explained
C．ExplainingD．It had been explained
D
本題考查時態(tài)。本題根據(jù)but he still couldn’t understand為過去時，故從句用過去完成時。句意：它解釋了一遍又一遍，但他仍無法理解。
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