i have passed 12th class in year 2012 scoring 77% in all subjects and 75%in P.C.M .since 40% of 12th class score will be now considered in deciding merit in engenerring entrance exam ,i want to give improvement in physics and chemistry in 2013.Can someone please tell me if somehow i get less marks as compared to my previous board exam then which marks will be taken as final.Also tell me can i give improvement for a particular subject like:-chemistry ,physicsetc or i have to give the improvement of all exams.