如图所示，虚线框内是磁感应强度为B的匀强磁场，导线框的三条竖直边的电阻均为r，长均为L，两横边电阻不计，线框平面与磁场方向垂直．当导线框以恒定速度v水平向右运动，ab边进入磁场时，ab两端的电势差为U₁，当cd边进入磁场时，ab两端的电势差为U₂，则（　　）

A．U₁=BLv

B．U₁=

C．U₂=BLv

D．U₂=

Kate had a terrible time this summer. Not a day _____ without having some unpleasantness with her sister at home.

A．she spent 　

B．she had spent

C．did she spend

D．had she spent