Renowned chemist Linus Pauling proposed a triple-helix structure for the DNA molecule shortly before James Watson and Francis Crick developed their double-helix model. In Pauling's model, three polynucleotide strands lined up with their sugar-phosphate backbones oriented together in the middle of the helix.

When Watson and Crick saw Pauling's model, they knew that it could not be correct. They realized that Pauling had not made use of a key piece of evidence about DNA that had been discovered by earlier investigations. What key piece of evidence did Watson and Crick apply to their model that Pauling did not apply to his?

A. Hershey and Chase's identification of DNA as the carrier of heritable information
B. Chargaff's finding that AT and GC ratios are constant across phyla
C. Griffith's finding that a nonliving substance could transform living cells
D. Levene's identification of the chemical structure of nucleotides

The famous biochemist and scientist Erwin Chargaff proposed two rules about DNA in 1950s just two years before Watson and Crick proposed the double helical structure of DNA.

The first rule proposed that in any DNA of cell, the amount of thymine units is equal to adenine units, and cytosine units is equal to guanine units. This rule gave a clue about pairing of bases in set of two.

The second rule proposed that amount of bases like guanine , adenine vary from one specie to another, this gave a clue that DNA is the genetic material and not proteins.

However, the information that Watson and Crick used from Chargaff's findings was his first rule. They thought that is bases adenine and thymine is equal it means that bases bind to each other in paired forms. So that each base is from one strand of DNA, making the overall DNA as double stranded.

Hope it help!