Like everyone has said, talk to your professor. He might be missing a grade.
Talking to them always works.
To note, he said a 71 was his lowest grade, not his average.
However also, you don't state what you think your average was. I like to point
out isn't grade school, you don't get points for showing up. You should attend all your
classes regardless. Showing up shouldn't improve your grade.
However I would also ask, if this class on a straight 10 point scale. I have had
classes were 83 was a C+.
I would just go and talk with your professor, and ask him why he gave you a C+
because you thought you were going to get/deserved a higher mark. Alot of the
times professors will explain there reasoning on why they felt you deserved the
mark they gave you, or can understand why you might deserve a higher grade.