∵电路元件A和B串联，∴IA=IB=0.2A，由图象可知：此时UA=2.0V，UB=2.5V；电源的电压U=UA+UB=2.0V+2.5V=4.5V；元件B的电功率PB=UBIB=2.5V×0.2A=0.5W．故选B．
