--Do you know when the Chinese custom_________ from?

--It's hard to say. But its characteristics__________ the Tang Period.

A．began; prove

B．started; show

C．dates; suggest

D．is; appear

采用单纯排放液化气的气体或液体均达不到降压的目的。必须（），才能实现降压。

A.先脱水

B.先行降温

C.排空

D.排污