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：本题考查宾语从句的连接代词。that 引导的宾语从句，若有两个或两个以上，只有第一个that可以省略。Having checked the doors were closed中省略了that。
【标签】名词性从句
