category index

例句：

1. 1. The library has a category index that helps users find books by topic.
   图书馆有一个按主题分类的索引，帮助用户找到书籍。
2. 2. The researcher used the category index to organize the data into categories.
   研究员使用分类索引将数据组织成类别。
3. 3. The teacher gave the students a category index to help them understand the structure of the text.
   老师给学生提供了一份分类索引，帮助他们理解文本的结构。