Select the phrases that accurately describe properties of the most common form of the dna double helix. a. the phosphodiester bonds between nucleotide residues run in opposite directions in the two strands. b. the sugar–phosphate backbone is exposed to the solvent, whereas the nitrogenous bases are in the interior of the double helix. c. base pairs have a spacing of . d. a helical turn consists of about . e. dna contains equal amounts of adenine and guanine (the purines) and equal amounts of cytosine and thymine (the pyrimidines).