如图所示，a和b是从A点以相同的动能射入匀强磁场的两个带等量电荷的粒子运动的半圆形径迹，已知其半径r_a=2r_b，则由此可知（   ）

    When you travel(旅游) in Hong Kong, you must be careful of the traffic(交通) rules, because the traffic keeps to the left and it's different from that of the interior(内地) of China. Before crossing the street, you must look to the right and then to the left. If the traffic lights are red, the traffic must stop, people can go across the zebra line.（斑马线） If the traffic lights turn green , the traffic can go, and people mustn't cross the road.
In the morning and in the evening, when people go to or come from work, the streets are very busy. The traffic is the most dangerous.
    When you go somewhere by bus in Hong Kong, you have to be careful, too. Always remember the traffic keeps to the left. Have a look first, or you may go the wrong way. In Hong Kong, there are a lot of big buses with two floors. You'd better sit on the second floor. From there you can watch the city very well. How interesting !

Here            your bag.