You can only be sure of __________ you have at present; you cannot be sure of something __________ you might get in the future.

A．that; what

B．what; /

C．which; that

D．/; that

进行氨的气密性试验，试验介质采用（）。

A.氮气

B.氧气

C.压缩空气

D.氨气