Ballet is not popular  our city because most people think it is too expensive to take the classes.

A．in

B．with

C．among

D．on

如图，垂直矩形金属框的匀强磁场的磁感应强度为B，导体棒ab垂直线框两长边搁在框上，ab长为l，在△t时间内，ab向右匀速滑过距离d，则

A．因右边面积减小ld，左边面积增大ld，则△φ=2Bld，

B．因右边面积减小ld，左边面积增大ld，减小磁通量与增大磁通量相互抵消，△φ=0，E=0

C．△φ=Bld，

D．因ab棒做切割磁感线运动，所以不能用计算感应电动势，只能用E=Blv计算感应电动势