Answer is C- The social life is as important as the athletic aspect in most masters.
C) your stressed is the answer
So its acquired immunity and the cells responsible are the B lymphocytes that produces antibodies against the virus. Memory is keep since first encounter of the virus by the immune system by way of the antibodies that was initially produced. The antibodies are there already, which makes it fast to act upon when expose again.
B) Move more quickly and frequently
Playing outdoor games together
hope you have a good day =)