Having checked the doors were closed and all the lights were off the boy opened the door

问题补充：

Having checked the doors were closed and all the lights were off, the boy opened the door to his bedroom.

A. why

B. that

C. when

D. where

答案：

A. why

(答案→)B. that

C. when

D. where

解析：句意：已经检查完门是关着的，所有的灯都灭了，这个男孩打开了他的卧室的门。这里逗号后是一个句子，逗号前是非谓语动词的形式，and连接的是Having checked的两个宾语，宾语从句不缺成分故用that，故选B。