Gonnif is spot on, not to mention the fact that given your prior record, that you are one exam in, and you scored 5 points above average on the first exam, I suspect you will be capable of pulling a b- or b in the class if you rebalance your efforts and let other coursework slip to low A range or even the upper b range. In any case a c is not the end of the world. I, for one, got a c+ in my general relativity class my sophomore year (curved up from a 48, ouch, which I spent about 80% of my time to achieve) and I have been fortunate enough to have interviews at three of the top five schools (whatever top five means but it illustrates my point). My gpa was not as high as yours either. Chill out and enjoy the standard of physics your teacher is holding you too; let it push you to a deeper insight into the subject; i wish I had a deeper understanding of electromagnetism than I do now as a result of a relatively easy algebra based physics course with which I was presented. I suspect holistically you will be successful in any case, particularly if you can take a breather given how gifted and hardworking you must be to have that gpa.
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