闭合开关S1、S2，电压表测R1两端电压，也就是电源电压为6V；电阻R1= U I1 = 6V 0.6A =10Ω，断开开关S2，此时电路电流I= U1 R1 = 2V 10Ω =0.2A，R2两端电压U2=U-U1=6V-2V=4V，R2= U2 I = 4V 0.2A =20Ω．故选B．
