The main idea presented in this excerpt is D. As fewer people get the flu shot, it becomes less effective, but as more people get it, the flu shot becomes more effective.
This refers to the central idea of a literary text which can also be the theme of the text.
Hence, we can see that from the given narration about the flu shots which were given, we can see that option D is correct as it shows the varying effects of the flu shot.
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https://brainly.com/question/163258