I _____ the prolem until he explaind it.A.understood B.did not understand C.had not understood D.

题目

I _____ the prolem until he explaind it.A.understood B.did not understand C.had not understood D.
had understood

答案

选B
not ……until 直到……才……
句意是：直到他解释了我才明白这个问题.