题目

在如图3所示的电路中，要使电阻R1R2串联，则应该    A．开关S1S2均断开                 B．开关S1S2均闭合    C．只要断开开关S1                 D．闭合开关S2断开开关S1 答案：A31．Tired of her endless complaints, I walked out of the room she could open her mouth．A．before B．until C．when D．once