Children should be encouraged to help their parents,which________helps them to learn how to

cooperate and share. [      ]

A. in return  

B. in addition  

C. in brief  

D. in turn

下列机床传动方式中，( )可以实现无级调速的运动。

A．齿轮传动

B．电子传动

C．气压传动

D．液压传动

E．蜗轮蜗杆传动