13．About 90 percent of the equipment presented by the administration，as well as the facilities of our own，      ______made good use of in teaching．

A．have

B．has

C．have been

D．has been

低压螺旋式气柜施工时，其施工的关键工序是( )。

A．气柜底板安装

B．水槽壁安装

C．顶板安装

D．塔体安装