Influenza Viruses a and B Contain Eight RNA Strands and Influenza

Influenza viruses A and B contain eight RNA strands and influenza virus C has seven strands.During antigenic drift, genes coding for glycoproteins recognized by the host immune system are mutated, whereas during antigenic shift, whole RNA strands can be swapped with those of another influenza virus, greatly increasing the virulence and transmissibility of the virus and minimizing the ability of the host's defenses to recognize it.Which of the following statements is true?

A) Genetic drift creates a need for an annual flu vaccine to keep up with the seasonal genetic changes, and if genetic shift occurs, an additional vaccine will have to be administered since the shift creates a different strainof virus.
B) Genetic drift creates a need for an annual flu vaccine to keep up with the seasonal genetic changes, and if a .genetic shift occurs, the same vaccine will be effective against both strains.
C) If a novel strain resulting from antigenic shift appears, it is not necessary to get vaccinated because these strains are typically less pathogenic than seasonal strains.
D) The worst influenza pandemics throughout history have occurred as a result of genetic drift rather than genetic shift.