Evolution of chemical matter and its level classification are not two irrelevant problems but two aspects of a problem.
化学物质的进化和化学物质的层次分类，是一个问题的两方面，而不是两个不相关的问题。
Yin Yang is the world outlook of "two-level structure", the five elements are classification of things.
阴阳是“二极结构”的世界观，五行是对事物要素的分类。
It is in classification of graduate student course, "door" is the top level of course, with two the code expresses.
在研究生学科分类中，“门”是学科的最高级别，用两位码表示。
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