汤姆森发现了____，说明原子是可分的，原子核是由 ____和____构成的，其中____带正

电 ，____不带电．

春华秋实七十载，守望相助谱新篇。2017年8月8日，内蒙古自治区成立70周年庆祝大会在内蒙古少数民族群众文化体育运动中心隆重举行。70年来特别是改革开放以来，内蒙古各项事业跨越式发展，取得了举世瞩目的辉煌成绩，经济实力显著增强，内蒙古生产总值增长642倍、人均增长近145倍，基础设施大幅改善，人民生活蒸蒸日上，实现了从温饱不足向全面小康迈进的历史性转变。

只用一种试剂或一种方法，就能区分氧气、二氧化碳和空气的是（   ）

根据短文内容，选择正确答案。
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.

计算，怎样简便怎样算．

15×4×25×6=____