If you honestly tried your best and those are the grades you got, then it's very sad because you are being punished for putting your best effort forward. It may be hard for your parents to understand that you did your best because they cannot see all the hard work you do at school in class.
Sadly, I think your parents are overreacting, too. I think that taking away things like your iPod and car is not appropriate for someone your age, especially for a reason like this! That kind of punishment would be appropriate for an 11 or 12 year old. I know this doesn't work because when I was a teenager, my father punished me the same way until I was 18. It did absolutely no good, except that without a car or anything to entertain myself all I did was sit on my butt and watch TV (if that wasn't taken away), which lead to being slightly overweight and socially unpopular with no extracurricular activities. That IS a punishment.
I'm very sorry that I can't give a definitive solution. Your parents are in a position of power and seem to be very stubborn on the subject, even though I think it's very unfair. They need to consider how many parents have to deal with children on drugs, drinking, having sex and getting pregnant, and kids who bring home Ds and Fs! I made a D in high school, heaven forbid, and I still got into a great college. The way kids behave now, you sound to me like the son everybody WISHES they had.
You could try having them look at this page, whether on purpose or "by accident", and perhaps seeing other peoples' opinions will change their minds.
Or, you could try getting them to help you with your homework. Half the parents now couldn't do what their kids do in school. My parents couldn't help me with my homework after 7th grade! Maybe if they see how hard it really is, they will back off of you.