如图所示，一电子从a点以速度v垂直进入长为d、宽为h的矩形磁场区域沿曲线ab运动，且通过b点离开磁场。已知电子的质量为m，电量为e，磁场的磁感应强度为B，ab的弧长为s，不计重力，则该电子在磁场中运动的时间为   ( )

A．t=

B．t=

C．t=arcsin()

D．t=arcos()

Beta blockers work on some physical and emotional symptoms because they ______.

A．fool a person into a healthier stance

B．interfere with the effects of adrenalin

C．produce side effects worse than the symptoms

D．primarily change human thought processes