I didn't manage to do it ---- you had explained how.A until

I didn't manage to do it ____ you had explained how.
这句话的意思是：直到你解释了如何做,我才做到.
这里应该是考not...until（直到...才）的用法.答案应该是A
如果选D
before如果后接从句,动词应该是过去式,因为before后面的动作应该是瞬间动作不能延续.此句里you had explained how用的是过去完成时,是延续的一种状态.因此不对