In my opinion,older people should________their own friends rather than become emotionally

dependent on their children.[ ]

A. spy  

B. seek  

C. serve  

D. stress

螺旋传动机构具有许多优点，但（）不是它的优点。

A、工作稳定

B、传动精度高

C、易于自锁

C、传动比大