如图，正方形区域ABCD中有垂直于纸面向里的匀强磁场，一个带正电

问题多选题

如图，正方形区域ABCD中有垂直于纸面向里的匀强磁场，一个带正电粒子（不计重力）以一定速度沿AB边的中点M 垂直于AB边射入磁场，恰好从A点射出．则（　　）

A．仅把该粒子改为带负电，粒子将从B点射出

B．仅增大磁感应强度，粒子在磁场中运动时间将增大

C．仅将磁场方向改为垂直于纸面向外，粒子在磁场中运动时间不变

D．仅减少带正电粒子速度，粒子将从AD之间的某点射出

答案

A、仅把该粒子改为带负电，由半径公式r=

得知，粒子的半径不变，向下偏转，则知粒子将从B点射出．故A正确．

B、仅增大磁感应强度，由周期公式T=

2πm

知，周期减小，粒子在磁场中运动半圈射出磁场，运动时间是半个周期，则知粒子在磁场中运动时间将减小．故B错误．

C、仅将磁场方向改为垂直于纸面向外，粒子运动的周期不变，粒子的半径不变，粒子向下偏转从B射出磁场，粒子在磁场中运动半圈射出磁场，运动时间是半个周期，故粒子在磁场中运动时间不变．故C正确．

D、仅减少带正电粒子速度，由半径公式r=

得知粒子的半径减小，偏转方向不变，则粒子将从AM之间的某点射出．故D错误．

故选AC

完形填空。
Look at the picture. It's a picture 1 a park. 2 the tree 3 some birds. Are there 4 boys under it? Yes, there are. They're 5 English books. 6 girls are there near the river? There're six. Are they 7 games? Yes, they are. 8 that girl behind the tree? That's Kate. She's 9 a red skirt. She's 10 tea.
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