题目

For kids like Edgar Ryan, who is recovering from a serious lung injury suffered this past summer, going out trick-or-treating is not a choice this Halloween. However, that didn’t stop him from dressing in a spooky(幽灵般的) costume and having some Halloween fun with the rest of the kids at The Children’s Hospital of Philadelphia, thanks to the organization Spirit of Children. The charitable arm of the Spirit Halloween stores, Spirit of Children will help more than 130 hospitals hold Halloween parties this year, each of which is funded by local donations. Since 2007, it has raise如图，在x＞0、y＞0的空间中有恒定的匀强磁场，磁感强度的方向垂直于oxy平面向里，大小为B．现有一质量为m电量为q的带电粒子，在x轴上到原点的距离为x0的P点，以平行于y轴的初速度υ射入此磁场，在磁场作用下沿垂直于y轴的方向射出此磁场．不计重力的影响．由这些条件可知（　　）A．不能确定粒子通过y轴时的位置B．不能确定粒子速度的大小C．不能确定粒子在磁场中运动所经历的时间D．以上三个判断都不对