In order to explain dna's consistent width, watson and crick concluded that a. one nucleotide will stretch its bonds to meet the other. b. one strand must run opposite of the other. c. one single ring base will always bond with one double ring base.

The answer is C. Purines (A and G) have double-ring structure while pyrimidines (C and T) have single rings. The obligate complimentary base pairing between a purine and pyrimidine ensures that the width between the helix backbones are maintained at approximately 20 angstroms.