What is the main cause of a human-created mass extinction? a. a decrease in the amount of ozone in the atmosphere b. an increase in greenhouse gases c. a decrease in the number of nonnative species introduced d. an increase in the human population

American scientist, linus pauling, was one of many scientists racing to discover the molecular structure of dna. pauling initially thought that the molecule was a triple helix. later, james watson and francis crick unveiled their double helix model, which turned out to be an accurate description of the molecule's shape. why did watson and crick succeed when pauling failed?