He went there, ______ some books and call on an old friend of his.

A．bought

B．buying

C．buy

D．to buy

气体继电器按动作原理不可，可分为两种结构，一是浮筒式气体继电器，二是挡板式气体继电器；按其接点形式不同，又有水银接点的气体继电器和干簧接点的气体继电器两种