如图所示：在真空中，有一半径为r的圆形区域内充满垂直纸面向里的匀强磁场，磁感应强度为B，一带电粒子质量为m，电量为q，以某一速度由a点沿半径方向射入磁场，从c点射出磁场时其速度方向改变了60度，（粒子的重力可忽略）试求

（1）该粒子在磁场中运动时间t

（2）粒子做圆周运动的半径R

（3）粒子运动的速度

Which of the following statements regarding the mark to market of a futures account is FALSE Marking to market of a futures account:()

A. may result in a margin balance above the initial margin amount and may be done more often than daily. 

B. is only done when the settlement price is below the maintenance price. 

C. effectively adjusts the price of the future to the new equilibrium level.