Analysis of the 1918 and 2009 flu epidemics reveals that the most likely scenario was that ________.

A) the virus causing these outbreaks were derived from a single strain within which significant mutations changed the protein hemagglutinin
B) these viruses switched from using RNA to using DNA
C) the virus ultimately arose in birds, jumped to pigs, and then was able to switch to humans
D) strong selection produces particularly virulent flu strains with predictable periodicity