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.
C. one single ring base will always bond with one double ring base.
Explanation;
At first, Watson and Crick thought the bases paired such that a like base pairs with its like (A with A, and so on), but such pairings did not result in a uniform width.
Instead, pairing a purine with a pyrimidine resulted in a uniform width consistent with the X-ray data.
Each base pair forms a different number of hydrogen bonds. Adenine and thymine form two hydrogen bonds, cytosine and guanine form three hydrogen bonds
