Suggested questions:

①How many class rules do you have?

②What must you do in the class?

③What mustn't you do in the class?

——How much money did you pay for the drink?
——None. It's ______.

FeSO₄+Na₂CO₃= FeCO₃↓+Na₂SO₄ 

FeCO₃+ 2CH₃CHOHCOOH =（CH₃CHOHCOO）₂Fe+CO₂↑+H₂O

如图所示，M为水平放置的橡胶圆盘，在其外侧面均匀地带有负电荷．在M正上方用丝线悬挂一个闭合铝环N，铝环也处于水平面中，且M盘和N环的中心在同一条竖直线O₁O₂上．现让橡胶圆盘由静止开始绕O₁O₂轴按图示方向逆时针加速转动，下列说法正确的是（   ）