It was not until she got home Mary realized she had left her books in the classroom. A. w

问题补充：

It was not until she got home Mary realized she had left her books in the classroom.

A. when

B. that

C. where

D. before

答案：

A. when

(答案→)B. that

C. where

D. before

解析：本题的it是代替句尾的主语从句，主语从句为陈述句，可用关系词that连接主语从句，故本题选B。