GMAT Reading Comprehension (RC) | Re: Sarah Meyers McGinty, in her useful new book Power Talk

I don't know why I have made one mistake for such a simple passage. I think I to handle all the questions very patiently before answering one.
For the last question I was confused in A and C. But I choose C because in the lines in the passage "communication skills as a way of preventing misunderstandings, avoiding conflict, and fostering interpersonal relationships. McGinty, however, holds that language not only helps maintain relationships but also lendsauthority[/b] " which is additional
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