维系天体系统之间关系的主要因素是（　　）

①万有引力  ②天体的运动  ③重力  ④自转．

    When you feel sad, tears will come down from your eyes. When you are happy, especially when you laugh hard, tears will also come down from your eyes. But tears have a more important job than showing your feelings.
    Tears keep your eyes clean and healthy. They wash away dirt and germs(细菌) just like bath. Your eyes also need tears to keep them wet. And eyes must be wet so that they can move smoothly. Your eyes are busy looking here and there all day long. They move quickly from one thing to another. If you didn't have tears, your eyes couldn't move, and soon you would be blind.
    Maybe you don't like tears, but your eyes can't do without them.

理解老师

刘静在一次篮球课上，A同学不小心碰到了B同学，于是两人争执起来。为了不影响其他同学练习，体育老师把他俩叫出来想单独解决，但是B同学转身就走到休息椅旁坐下对我的话不予理睬。