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D
    The students are having their chemistry (化学) class. Miss Liu is telling the children what water is like. After that, she asks her students, “What's water?”No one speaks for a few minutes. Miss Liu asks again, “Why don't you answer my question? Don't I tell you what water is like?”
    Then a boy puts up his hand and says, “Miss Liu, you tell us that water has no color and no smell. But where can we find such kind of water? The water in the river behind my house is always black and it has bad smell. "Most of the children agree with (同意) him.
    “I'm sorry, children.” says Miss Liu. “Our water is getting dirtier and dirtier. That's a problem. "