Explain how the work done by rosalind franklin and maurice wilkins inform watson and cricks model of the structure of dna

Rosalind Franklin and Maurice Wilkins conducted experiments using X-ray crystallography to study the structure of the DNA. Their experiments produced the famous photo 51, the first evidence on the structure of the DNA. Their work showed that DNA is double-stranded and forming a helix. They also showed that the nitrogenous bases are on the inside of the strands and that the phosphates are on the outside. Watson and Crick used these observations to develop their model on the molecular structure of the DNA. 

The answer would be c