《翻身农奴把歌唱》唱到：“驱散乌云见太阳”，西藏人民摆脱了帝国主义的侵略和奴役，祖国大陆实现了统一，指的是（    ）

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      Do you need friends? I'm sure your answer is "Yes, of course. Everybody does! "You need friends when you play and when you work. If you have friends, you will feel happy. If you have no friends, you will feel lonely.

      Do you know how to make friends? There is only one good way—You make friends by being friendly.

      A friendly person is interested in other people. He is always helpful. If you want to make friends with a new classmate, you can talk with him, tell him about the other classmates in your class and try your best to be helpful to him.

用计算器计算：

（1）$\sqrt{9\times 9+19}$=

（2）$\sqrt{99\times 99+199}$=

（3）$\sqrt{999\times 999+1999}$=

（4）$\sqrt{9999\times 9999+19999}$=

观察上面几题的结果，你能发现什么规律？用你发现的规律直接写出下题的结果：

 =

家兔是进行有性生殖的生物，之所以能保持前后代染色体数目的恒定，是因为在生殖过程中进行