发电机发电时，其电源输出的（）必须与原供电系统一样。
A.电压
B.相序
C.相位
D.电流
参考答案：B
They bring gifts _____________ us.
A. by
B. for
C. of
There’s a ________ in our office that when it’s somebody’s birthday，they bring in a cake for us all to share. (2011·山东，34)
A．tradition
B．balance
C．concern
D．relationship
