The central idea that's in a story simply means the unifying element that is contained in the story.
Your information is incomplete as you didn't give the passage or story. Therefore, an overview will be given. The central idea is the main idea that's in the story.
The central idea is the most important idea that the author is communicating to the reader. In this case, it's important to read and understand the story.
Also, understand the themes and plot in the story. Read paragraph 15 carefully to understand the main idea that the author wants to pass across to the readers.
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