I can tell you from experience that a D is in fact a passing grade

At my college we have, like many [liberal arts] schools, certain types of courses we must take in order to graduate. I'm not a fan of these courses. My last semester, I took my course in Ethics (in the Philosophy department). Not only had I never taken a course like this before, but apparently I was taking it with the worst professor I could have chosen. Just to give you an idea of how hard it was, I did more reading and work for that class then any other, and on the first exam I recieved a 32% grade. What's funny is that this translated to a D! In the end, I was worried whether they would accept a final grade of D for this required course, and so I didn't know if I would grduate from college until 2 days before commencement!! That wasn't fun

Good luck with yours, though. That sucks. However I agree that it's best to send the professor a polite email and see what happens. The thing I was best at in my entire school career was my relationships with my teachers. I was extremely polite and it got me further than I can imagine.