启闭机按活塞杆受力状况分为单向作用与双向作用。

A、液压式

B、螺杆式

C、固定卷扬式

D、移动式

Engineering is ______ challenging course of study that requires ______ thoroughunderstanding of mathematics and science.

A．the; a

B．a; the

C．不填；不填

D．a; a