Initial studies of the influenza A virus by Walter Fitch and colleagues showed that ________.

A) influenza A actually has DNA as its genetic material
B) selection is causing influenza A to become more virulent over time
C) selection by the immune system drives change in hemagglutinin's antigenic sites
D) protein evolution in influenza A is consistent with the neutral theory
E) flu strains from one host cannot exchange genetic material with strains from another