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.
）在一定条件下可以发生如下图所示的转化（其他产物和水已略去）．