单项选择 —These bananas are about 5 kilos______ total. Would you like to buy them? --Sure. How much are they? A. for B. on C. in D. at 答案:C一个质量为m的带负电量q的油滴从高h处自由落下,下落h后进入一个正交匀强电场和匀强磁场叠加区域,匀强电场方向竖直向下,磁场磁感应强度为B,进入场区后油滴恰好做圆周运动,其轨迹如图所示,由此可知匀强电场场强大小E= ,所加磁场方向是 ,油滴作圆周运动的半径R= 。