A:A球与C球一定带有异种电荷
B:B球与D球一定带有同种电荷
C:B球可能带电,也可能不带电
D:D球可能带电,也可能不带电
—I often go to school on ___ bike. But this morning I took a bus.
关于电流的方向,下列说法中不正确的是( )