Only after I read the text over again _____ its main idea.

Only after I read the text over again _____ its main idea.
A、that I knew
B、did I know
C、I could know
D、I did know
【正确答案】：B
【题目解析】：当only +状语位于句首时。要引起部分倒装，knew 为谓语动词，因此要借用助动词did , B选项正确。