化工管路在投入运行之前，必须保证其强度与严密性符合设计要求。当管路安装完毕后，必须进行压力试验，称为试压。除特殊情况外，试压主要采用(　　)试验。

A．冲击

B．负压

C．气压

D．液压

Stamp collection has long been recognized as having an educational value. There is every reason to encourage young people to take an interest in them, thereby ______ to their knowledge of geography and history.

A．to add

B．adding

C．added

D．adds