∵两电阻并联，∴电源的电压U=U2=U1=I1R1=0.3A×10Ω=3V；R2的电流I2=I-I1=0.5A-0.3A=0.2A，R2= U2 I2 = 3V 0.2A =15Ω；电阻R1消耗的电功率P1=U1I1=3V×0.3A=0.9W．故选B．
