Scientists build models based on what they know from previous research to derive testable hypotheses. Independently, both Watson and Crick and their competitor Linus Pauling constructed an incorrect triple helix model with the nitrogenous bases arranged so they were on the exterior of the molecule and the phosphate groups on the interior. Although their model was wrong, what assumption made it reasonable to build a model with the bases projecting to the outside

Nitrogenous bases contain the genetic information, their amount is variable among different species, and the arrangement of these bases is also variable among different species

Explanation:

Both Watson-Crick and Pauling's DNA models considered that DNA nitrogenous bases (i.e., Adenine, Cytosine, Thymine and Guanine) contain the genetic information that determines the characteristics of living organisms. Moreover, both DNA models also considered that nitrogenous base composition varies between species, as well as the arrangement of these bases in the DNA chain also varies between species. Based on these features, Linus Pauling considered that a model where nitrogenous bases would be arranged on the outside of the DNA molecule would be easier for the DNA molecule to be replicated, transcribed, or repaired. Although incorrect, Pauling's DNA triple helix model was fundamental to develop the helical (double-stranded) structure of DNA, which was finally discovered by Watson and Crick in 1953.