If I were you (I was a graduate student in a couple of programs, and I'm now a teacher), I would keep a professional distance from C. It's the professor's job to inform C. if C. doesn't understand the course schedule or assignments. Make the professor do her/his job. Some graduate students can be difficult to have in class or do group work with due to competitiveness, pettiness, stupidity, nervousness, anti-sociability. Just go with the flow and don't be that guy. The more you can rise above the fray, the more you will be respected by your fellow students and professors. If C. disagrees directly with one of your ideas or criticizes you directly for some reason, then you should certainly respond directly and defend yourself. But only engage directly if you are assigned to by the professor or C. clearly attacks one of your statements, ideas, or presentations.