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.

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      刘崧，字子高，泰和人，旧名楚。家贫力学，寒无垆火，手皲裂而钞录不辍。崧幼博学，天性廉慎。兄弟三人共居一茅屋，有田五十亩。及贵，无所增益①。十年一布被，鼠伤，始易之，仍葺（qì）以衣其子。居官未尝以家累②自随。之任北平，携一童往，至则遣还。晡（bū）时吏退③，孤灯读书，往往达旦。善为诗，豫章④人宗⑤之为“西江派⑥”云。

     【注释】①增益：增加。②家累：家属。③晡时吏退：晡时是指下午3时正至下午5时正；吏退是指下班。④豫章：今南昌。⑤宗：崇尚，推崇。⑥西江派：明初江右诗派，刘崧为该诗派的代表人物。

太阳直射最北、最南的界线是（   ）

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