—Would you like me        the radio a bit?

—No, it’s all right. I’m used        with the radio        .

A．to turn down; to work; on

B．turning down; to working; on

C．turning down; to working; off

D．to turn down; to working; on

在图所示的电路中，电阻R=2.0Ω，电源的内电阻r=1.0Ω，不计电流表的内阻．闭合开关S后，电流表的示数I=0.5A，则电源的电动势E等于（　　）

A．1.0V

B．1.5V

C．2.0V

D．3.0V