One of the main themes of the summit was “sustainable development”,or the question _________ we can continue developing the world without damaging the environment.

A．what

B．how

C．why

D．that 

若希望实现三相异步电动机的向上向下平滑调速，则应采用（）。

A.串转子电阻调速方案 

B.串定子电阻调速方案 

C.调频调速方案 

D.变磁极对数调速方案