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

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I_____theprolemuntilheexplaindit.A.understoodB.didnotunderstandC.hadnotunderstoodD.hadunderstoodI___

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

I _____ the prolem until he explaind it.A.understood B.did not understand C.had not understood D.had understood
选B
not ……until 直到……才……
句意是:直到他解释了我才明白这个问题.