16.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

【解析】B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。