A:13
B:12
C:25
Stephen Hawking believes that earth is unlikely to be the only planet life has developed gradually.
电路如图所示,当开关S1闭合,S2断开时,电压表的示数为2.5V;当开关S1 断开,S2闭合时,电压表的示数为6V,则 灯泡L2两端的电压为( )