—Shall I turn on the TV, Mom?

—No. You should be quiet _____ wake up your sleeping brother.

A．so not as to

B．so as not to

C．so as to not

D．not so as to

如图所示，空间分布着宽为L、场强为E的匀强电场和两磁感强度大小均为B、方向相反的匀强磁场(虚线为磁场分界线，右边磁场范围足够大)．质量为m、电量为q的离子从A点由静止释放后经电场加速进入磁场，穿过中间磁场后按某一路径能再回到A点而重复前述过程．求：

(1)离子进入磁场时的速度大小和运动半径．

(2)中间磁场的宽度D．