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

雨水大量渗入渠堤内，使土层软化饱和，降低了渠堤的（），形成滑坡。

A、抗滑能力

B、抗渗能力

C、抗压强度

D、抗冲能力