The Reason That Vaccines Against Influenza Must Be Administered Annually,unlike

The reason that vaccines against influenza must be administered annually,unlike vaccines against measles,is _____.

A) the antigens that stimulate protection against influenza virus are inside the virion and not on the surface
B) influenza is an RNA virus with a higher mutation rate
C) influenza stimulates T-independent responses that fail to generate memory cells
D) the polysaccharide antigens of influenza stimulate poor immune responses.