Well done Samantha Nixon, they're really great. I wouldn't be too worried about the D. Like you said, you can resit it if you want next year, but as the Science modules go on and you sit more exams in that module, say if you get 2 Cs and a D, the 2 Cs would cancel out the D giving you a C overall.
In the exams you just got the results back from, if you got a high C and a high D, then if you mix the results together, you'd get a C overall. Hope that makes sense!