—He is the only one of the students ______from the country in my class, and we have known for quite some time.

—Where did you meet for the first time?     

—It was on the farm ________ I used to work.

A．who are; that

B．who is; where

C．that is; that

D．whom are; on which

当电力系统频率降低时，负荷的频率调节效应（）。

A.可以使负荷的功率降低

B.可以使负荷的功率增加

C.可以使负荷的功率不变

D.可以使负荷的功率周期变化