From the passage, we know that web directories are _________.

C
逻辑分析题。由文章第二段第二句“Web dictionaries can have a general or topic specific focus and are usually organized into several categories based on the topic specific focus．”(网页目录有普遍或特定的焦点范围，它是根据特定的话题类型组织起来的。)可知选项C(arranged based on the topic根据话题编排)与文意最为接近，因此C为正确答案。