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.

俄罗斯境内的石油、天然气运输主要是（   ）

—Don't worry. There is still        time left.

通过呼吸运动，肺实现了与外界环境的气体交换．下列关于平静呼吸的表示，正确的是（　　）