Only then _____that I had made another mistake .But it was too late

题目

Only then _____that I had made another mistake .But it was too late
A:did i realize B:i realized C:i was realizing D:was i realize为什么要选A呢?

答案

only then 要动词前提,类似虚拟语气的做法.所以did前提,后面用动词原型