It was after he went to college he realized the importance of forming a reading habit.A.

问题补充：

It was after he went to college he realized the importance of forming a reading habit.

A. that

B. when

C. since

D. as

答案：

(答案→)A. that

B. when

C. since

D. as

解析：本题考查强调句型。将It was和空格去掉后剩下的部分句意和结构依然完整，所以题干是个强调句型，故选A。