题目

阅读理解Ask people in the UK whatthe words Sunday roast mean to them, and they'll probably take youback to their grandmothers' dining rooms - maybe with a few stories of thegreatest puddings and the best ever steak. But now thetraditional Sunday roast seems to have been left back in the old days.According to the Daily Mail, just one in 50 British familiessits down to this weekly meal together.There are many reasons whythe roast is becoming less popular. In the busy modern world, where breakfastis a slice of toast eaten on the way to work or school and lunch is a quicksandwich in front of t如图所示，带有正电荷的A粒子和B粒子同时从匀强磁场的边界上的P点以等大的速度，以与边界 成30°和60°的交角射入磁场，又恰好不从另一边界飞出，设边界上方的磁场范围足够大，下列说法中正确的是  （    ） A. A 、B两粒子在磁场中做圆周运动的半径之比为 B. A 、B两粒子在磁场中做圆周运动的半径之比为 C. A 、B两粒子的之比为     D. A 、B两粒子的之比为