—Thanks for your help, but can you do me one favor, please?

—___________, sir. What's it then?[ ]

A. Just a minute      

B. My pleasure      

C. That's all right      

D. At your service  

如图所示为某宾馆的房卡．只有当房卡插入卡槽内．房间内的灯和插座才能有电。房卡的作用相当于一个______（填路元件名称）接在干路上．插座、电视等用器，它们是______联．