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

the door to his bedroom.   [ ]

A. that            

B. why            

C. when              

D. where

多煤层赋存的煤层群开采，原则上按照（）的顺序开采。

A、自上而下

B、自下而上

C、同时开采