在异步电动机的调速系统中，效率最高、性能最好的系统是（）调速系统。
A.变极
B.变频
C.变转差率
D.以上选项均不正确
参考答案：B
—My grandma decides to _______ her company to me after she is retired.
—You must be under great pressure.
A．take over
B．hand over
C．go over
D．run over
已知下面三个数的平均数是170，请你在空缺的横线里填上正确的数．
______；______9；______26．
