下列哪一项不属于中枢神经的组成

（x﹣y）⁹的展开式中，系数最大项的系数是（   ）

硝酸钾（KNO₃）粉末中含有少量杂质氯化钾（KCl）．回答下列有关问题：

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A
    My name's Li Yan．I'm from Belling and I'm 13 years old.I study in No．1 Middle School．I'm in Class 2．Grade 7．I'm in the school music club．I can play the guitar well and I am good at playing the piano．Lang Lang is my favorite musician．I dream of being a musician like him．
    LiYue is my sister．We are twins(双胞胎)．We go to the same school， but she is in Class 4．Grade 7．She can play ping-pong very well and she is in the sports club．She likes Zhang Jike and Ma Long best．She often watches them play ping-pong on TV.
    My sister and I are learning how to swim．We are not good at it now,but we are trying our best(尽我们最大的努力)to learn it．It is a really healthy sport．
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