The main idea of the announcement is: "an author will talk to the students about his new book" (Option B). Another name for the main idea is the central idea.
The main idea is the principal thought that the author wishes to communicate. The phrase "Main Idea" is suggestive of the fact that there are other ideas in the text.
The main idea is the idea that unifies all the elements of the text. It is usually supported by all other ideas in the text.
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