It was not until the teacher had explained it three times how to work out the problem.A.

问题补充：

It was not until the teacher had explained it three times how to work out the problem.

A. did he understand

B. had he understood

C. that he understood

D. that he didn’t understood

答案：

A. did he understand

B. had he understood

(答案→)C. that he understood

D. that he didn’t understood

解析：考查固定句型。句义：直到老师解释了三次我才明白了怎样做这个题，首先给我关键词：It was …+that …,构成强调句型，根据倒装句：not until 位于句首构成：主到从不倒；用语强调句型不倒装，所以C正确。