Why do cells have a plasma membrane? a) to separate protiens from the nucleus b) to distinguish plants from animals c) to define the size of the cell d) to separate the inside of the cell from its environment plzz well make brainlest plz answer in less than 10

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?