In 1953, Watson and Crick were able to propose a model of DNA as a helical molecule with a sugar-phosphate backbone on the outside of the molecule. Which of the following earlier discoveries was most instrumental to this important discovery? By studying this photograph, scientists gained knowledge about the —
A. the discovery by Hershey and Chase that only DNA from a virus need enter a bacterial cell to cause an infection
B. X-ray crystallography studies of DNA conducted by Rosalind Franklin in 1951
C. the identification of DNA as the agent of bacterial transformation by Oswald Avery
D. the 1948 discovery by Linus Pauling that proteins take the shape of an alpha helix