(2010•衡阳)如图所示,电源电压不变,当开关S闭合时,电流表的示数为0.8A.开关S断开后,电流表的示数改变了0.5A.则R1与R2的阻值之比为()
A.13:5
B.3:5
C.5:3
D.5:13