一台理想变压器的原、副线圈匝数比为4:1，若原线圈加上311V的直流电，则用交变电压表测得副线圈两端的电压是（     ）

A．250V

B．220V

C．0V

D．200V

During the day，a dredge will indicate the side on which it is safe to pass by displaying（）.

A.two balls in a vertical line

B.two diamonds in a vertical line

C.a single black ball

D.no shape is shown during the day