What are we answering here there is no question or thing to do?
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trust me, i just answered this question on i-Ready :) so i guarantee this is correct.
The correct answer is C. For the first time in decades, Baby Boomers have been outnumbered by another group--the Millennials.
A hook is the sentence that metaphorically 'hooks' you - that is the most engaging sentence in the introductory paragraph that makes you interested in what is going to happen next. In this paragraph, option C is the hook because we are about to find out how the Millennials managed to replace Baby Boomers.