—  Have you ever been to the city of Beijing, our capital?

— Yes, only once. I ______ there only for two days.  [      ]

A. had stayed  

B. were staying

C. have stayed      

D. stayed

在匀强电场和匀强磁场共存的区域内，电场的场强为E，方向竖直向下，磁场的磁感应强度为B，方向垂直于纸面向里，一质量为m的带电粒子，在场区内的竖直平面内做匀速圆周运动，则可判断该带电质点

A．带有电荷量为的正电荷

B．沿圆周逆时针运动

C．运动的角速度为

D．运动的速率为