Great changes have taken place in that school. It’s no longer ___ it was 20 years ago, ____ it was so poor.

A．what, when

B．that, which

C．what, which

D．which, that

强制送风呼吸器的工作原理是通过送风机使被污染的空气经滤毒装置过滤成可供正常呼吸的洁净气体，由呼吸管进入面罩供使用者呼吸，呼出的气体通过排气阀排到大气中。