如图所示,电源电压保持不变,R1=10Ω,R2=15Ω。当S闭合时,电压表的示数为6V,则电源电压为( )
A:10V
B:7.5V
C:3.5V
D:2.5V
— I'm not going swimming this afternoon.— ______. I have to help my mother do some cleaning.
散:sàn____ sǎn____
落:luò____ lào____ là____
弹:dàn____ tán____
乐:lè____ yuè____