无功补偿最有利的补偿方式是（）。
A.个别补偿
B.分组补偿
C.集中补偿
参考答案：A
楼梯升降机工作时蜂鸣器会（），红色信号灯会亮。
— I rang you at about nine, but there was no reply.
— Oh, that was probably _____I was seeing the doctor.
A．why
B．when
C．what
D．That
