The Explanation for C

I got C right because it is true that Y undermines S's argument by pointing out that alcohol problems occur even at universities where there are no fraternities. I thought that in saying that, Y does address S's point because S says in schools where there are no frats, there is no alcohol and Y directly addresses her point by saying that all schools - frat or not - have an alcohol problem. So I'm confused as to why the explanation is saying that she isn't addressing S's point at all.