我国人与地突出的问题是（　　）

两个带电粒子以同一速度，并从同一位置进入匀强磁场，在磁场中它们的轨迹如图所示（完成两个半圆运动）．粒子的轨迹半径为，粒子的轨迹半径为，且，、分别是它们的带电量，则 （      ）

如图是20世纪初西方报刊上的一幅时局漫画《慈禧太后跑路》，关于这一时局的正确说法是（  ）

— What does that man look like?

— He's of medium height and very ____.

______ they live in the same house, we would no trouble finding them.
---But as traveling salesmen as well as seasonal herdsmen and farmhands, as you know, they move house _______ many of the Americans who live on wheels.