He chose to teach in a western province, though he ______ in the city for a better life. [ ]

A. could stay

B. would stay

C. could have stayed

D. had stayed

能使运动部件实现往复直线运动，且往复速度不一样的是（）油缸。

A.双杆活塞

B.单杆活塞

C.柱塞式

D.双杆柱塞式