—Excuse me, may I take a seat here?

—________ The man on the seat has already left. [ ]

A. You'd better not.

B. Yes, please.

C. No, you can't

如图所示为某宾馆的房卡．只有把房卡插入槽中，房间内的灯和插座才能有电．房卡相当于一个接在干路上   （填写一个元件名称）。房间里有电灯、电视等用电器，它们是   联的．