My parents are so critical of my grades!?

hello. im tired of my parents being so uptight about my grades! i make all A's and B's (usually high B's). i just recieved my progress reports and i have all A's and B's. so i get home and my stepmom has to question every bad grade on my freakin progress report! i had 1 or 2 bad grades (c and d) on each report and an F on one. the thing is, is that those low grades were on little ignments and most of my ignments are A's! why can't they just back off and let me make my grades. im trying as hard as i can because i really want to go to U.T but they just wont stop! i have an 81 in english (im deffinatly bringing that one up) and my parents are like "you better bring that up!" wtf! sorry if it seems like im ranting but im sick of this! im not perfect! im gonna screw up every now and then and the last thing i need is for them too tell me about it. also i dont think my stepmom has any right what so ever to complain to me about my grades. shes not my mom so she can shove it! so what can