已知$\left\{\begin{array}{c}x=2\\ y=1\end{array}\right.$是二元一次方程组$\left\{\begin{array}{c}mx+ny=8\\ nx-my=1\end{array}\right.$的解，则2m﹣n的平方根为（　　）

铁的氧化物可用于脱除煤气中的H₂S，有一步反应为：Fe₃O₄（s）+3H₂S（g）+H₂（g）⇌3FeS（s）+4H₂O（g）其温度与平衡常数的关系如图所示．对此反应原理的理解正确的是（　　）

        These days, many students like playing 3D games. Do you know what 3D games are like? In most 3D games, game players need to control (控制) a character. The character can be a robot or a soldier (士兵). Game players usually use the mouse to make the character move around in his own world. Game players can find things such as food and weapons (武器) to help them go on with the game. The character can go straight, sit down, turn left, turn right or pick up things in the game.
        Some 3D games have many levels (等级). The character needs to finish different goals for each level. Game players can play against their computers, and sometimes they can play with other players online. It’s great fun to play 3D games. But playing 3D games for long is not good for our study.

如图（甲）所示，质量不计的弹簧竖直固定在水平面上，时刻，将一金属小球从弹簧正上方某一高度处由静止释放，小球落到弹簧上压缩弹簧到最低点，然后又被弹起离开弹簧，上升到一定高度后再下落，如此反复。通过安装在弹簧下端的压力传感器，测出这一过程弹簧弹力随时间变化的图像如图（乙）如示，则

为迎接2008年的“绿色奥运”，首都绿化委员会办公室确定了100多种树木为京城绿化的“主力军”，这些造林树木中，不可能有（ 　）