根据板块构造学说,地中海的面积将会越来越小,直至消失,原因是( )
A:板块碰撞运动
B:板块扩张运动
C:地壳下沉运动
D:流水运动
— He's never advised to achieve a balance between his job and hobbies, ________? — _______. His parents and teachers often offer him advice.
如图所示,一种射线管由平行金属板A,B和平行于金属板的细管C组成.放射源O在A极板左端,可以向各个方向发射不同速度、质量为m的电子.若极板长为L,间距为d,当A,B板加上电压U时,只有某一速度的电子能从细管C水平射出,细管C离两板等距.已知元电荷为e,则从放射源O发射出的电子的这一速度为( )