当开关S1与S2闭合，S3断开时，I=0.6A，R1=3Ω，故可求得电源电压为U=I?R1=0.6×3V=1.8V．当开关S1与S3闭合，S2断开时，总电阻R= R1?R2 R1+R2 = 3Ω×6Ω 3Ω+6Ω =2Ω，故此时的电流为I= U R = 1.8V 2Ω =0.9A故选A．
