6. (a). Believe or not protein was thought to be a genetic material. This thought was first be challenged in1920s by Frederick Griffith’s experiment and in1943 by Oswald Avery, Colin Macleod, and Maclyn McCarty’s experiment. Please briefly describe the experiments done by Avery et al and their conclusion. (b). Why it was hard to convince that DNA is the genetic material in those days? (Your answer must be based on the structural difference between DNA and protein and their implication to comment on this question)