In the watson-crick model of dna structure: a. nitrogenous bases are covalently attached to carbon 1' of deoxyribose t can form three hydrogen bonds with either g or c in the opposite strand. b. the distance between the sugar backbone of the two strands is just large enough to accommodate either two purines or two pyrimidines. c. both strands run in the same direction, 3' -> 5'; they are parallel. d. phosphate groups project toward the middle of the helix, where they are protected from interaction with water.