When identifying the central idea of a tect, it is most helpful to notice the

repetition of ideas

Explanation:

When you can identify the central idea of a text, it is helpful to note the repetition of ideas within the same text so that you can interpret the text succinctly, concretely and comprehensively. When you observe the repetition of ideas you will begin to fully understand what the author wants to get through with the written text.

- "the basic difference between keywords and subjects is that subjects (or "subject headings") have been predetermined, while keywords have not. a keyword is an easy place to start because you can use any word that seems to fit your search; this is called "natural language" searching."