A:by accident
B:by mistake
C:all of sudden
D:without doubt
如图所示,间距为L的平行金属导轨上,有一电阻为r的金属棒ab与导轨接触良好.导轨左端连接电阻R,其它电阻不计,磁感应强度为B,金属捧ab以速度v向右作匀速运动,则( )
You will be late for school ________ you don't get up early.