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.

向下列各物质的水溶液中滴加稀硫酸或氯化镁溶液时，均有白色沉淀生成的是（　）

北极的极地狐耳朵小，沙漠的大耳狐耳朵大．造成这一差异的主要非生物因素是（    ）