下图所示的五角星是用螺栓将两端打有孔的5根木条连接构成的图形，它的形状不稳定。如果在木条交叉点打孔加装螺栓的办法使其形状稳定，那么至少需要添加(      )个螺栓。
 

热敏电阻的阻值随温度的改变而改变，且对温度很敏感，热敏电阻的阻值在特定温度时会发生急剧变化，它在笔记本电脑、自动控制系统等方面有多种用途．如甲电路图，电源电压是9V．当闭合开关S，电压表示数如乙图所示，热敏电阻的电流随电压变化的I～U关系如丙图所示．求

—Who is ____(躲) behind the door?

—Sorry, I don't know.

下列叙述正确的是                                                （   ）

    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 on-line. It's great fun to play 3D games. But playing 3D games for long is not good for our study.