Tom is ________ his pen, but he can't ________ it. Let's help him.
A:find; look for
B:looking for; find
C:look for; find
D:finding; look for
如图所示,以MN为界的两匀强磁场,磁感应强度B1=2B2,方向垂直纸面向里,现有一质量为m、带电量为q的正粒子,从O点沿图示方向进入B1中.
(Ⅰ)求椭圆的方程;
(Ⅱ)直线l不经过点A(0,1),且与椭圆交于M,N两点,若以MN为直径的圆经过点A,求证:直线l过定点,并求出该定点的坐标.