There are two main classes, Type I and Type II.

超新星有两类：I型和II型。

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The main classes are RootDocs that contain the parse tree.

主要的类是包含解析树的RootDoc 。

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Those problems fell naturally into several main classes.

这些问题可自然地分为几大类。

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Those problems fell naturally into several main classes.

这些问题可自然地分为几大类。

youdao