＿＿＿the people have become the master of their own country＿＿＿science can really serve the people.

A．It is only then； that

B．It was that； when

C．It is only when； that

D．It was when； then

在一定的井深、压力下，水泥浆允许水或气体通过的能力称为（）。

A、含水率

B、含水率或含气率

C、水泥浆渗透率

D、通过率