Which of the following statements is NOT true about antigenic drift for influenza?
O a. involves mutations in genes encoding the H and N spikes
Ob. allows the virus to avoid IgA antibodies at the mucosal membranes
Oc. probably due to genetic recombination between different strains infecting the same cell
O d. involves minor changes in the virus's surface proteins
Oe. may involve as few as one amino acid that is changed