The central idea is the central, unifying element of the story, which ties together all of the other elements of fiction used by the author to tell the story. The central idea can be best described as the dominant impression or the universal, generic truth found in the story. Therefore that is why that is the central idea
Should be D, adapted. I don't see how the fish enlarged or departed anywhere, and it didn't waver, but it has a special adaptation to the cold, so the best answer is adapted.
