It was not until he arrived at the office realized he had forgotten the key. A. and heB.

问题补充：

It was not until he arrived at the office realized he had forgotten the key.

A. and he

B. when he

C. that he

D. he

答案：

A. and he

B. when he

(答案→)C. that he

D. he

解析：本题的含义是他到达办公室他才意思到他忘记了钥匙，本题的主语用it来代替，真正的主语从句位于句尾，陈述句通常用that连接主语从句，故本题选C。