题目

―Hurry, John!  ―Oh, damn! Both my legs are nearly to give out. I _____ for hours.  A. had walked               B. walked                     C. have been walking    D. am walking   答案：C   10．在如图所示的电路中，电源的内电阻r不能忽略，其电动势E小于电容器C的耐压值．开关S原来是断开的，现闭合开关S，则在电路再次达到稳定的过程中，下列说法中正确的是（　　）A．电阻R1两端的电压减小B．电容器C两端的电压增大C．电源两端的电压减小D．电容器C上所带的电量减小