don't know who wrote this, but you should have taken this up higher since this is completely inappropriate. a professor has to _prove_ plagiarism, you don't have to _prove_ that you didn't do it.
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It was me, and it was forty years ago. I thought about it and decided some battles are not worth the effort. English didn't mean shit to me. But the English story wasn't over. Next I transferred to Univ of Iowa and they didn't accept that English course from Cal State Northridge as fulfilling their English requirement, so I took yet another freshman comp class---only on that round I took the rhetoric or speech version---same grade C.
In the end after four rounds on the very same basic requirement, I decided that it was actually good, since I had a lot of practice. It was also obvious that grades in this particular subject were arbitrary.
At a certain point you realize that academia is some kind of certification proceedure and has nothing to do with an intellectual life. So my odyssey through the bowels of English departments was part of that lesson.
Chuck Grimes