题目

—Good morning. Room Reservations. May I help you, sir?     —Yes, I’d like to book a room.     —            Which date would that be?     A．Thank you, sir.                             B．That’s all right, sir.     C．You are welcome, sir.        D．No trouble,sir.   答案：A如图所示电路，电源电压保持不变。闭合开关S，当滑动变阻器的滑片P从a端滑到b端的过程中，R1、R2的U－I关系图象如图所示。则下列判断正确的是（ ）①图线甲是电阻R1的“U－I”关系图象②电源电压为12V③电阻R1的最小电功率为2.7W④滑动变阻器R2的最大阻值为30ΩA. 只有①③正确 B. 只有①④正确C. 只有②④正确 D. 只有②③正确