It was after she had made a thorough investigation ____she came

It was after she had made a thorough investigation ____she came to know the actual state of affairs.
A、then
B、that
C、so that
D、therefore
【正确答案】：B
【题目解析】：B她是在进行彻底的调查之后，才逐渐了解事件的真相解析：很明显此处为 It was ... that...强调结构。