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
网友回答
A. that I knew
(答案→)B. did I know
C. I could know
D. I did know
解析：考查倒装句。此题为only + 状语放在句首使用部分倒装，题中加的是一个时间状语，句意：只有我再次读完这篇课文之后我才能够知道它的主要意思。所以选B。