How Does Antigenic Drift Explain the Need to Get a New

How does antigenic drift explain the need to get a new flu vaccine every year? (Select all that apply)

A) antigenic drift is the process in which the primary viral antigens like H and N proteins slowly accumulate mutations
B) H and N mutations become so numerous that the resulting proteins cannot be recognized by previously produced antibodies
C) the current vaccine which was developed using the last year's viruses stimulates production of IgG antibodies that will recognize most H and N proteins but a booster is needed to keep IgG levels elevated
D) the current vaccine which was developed using the last year's viruses stimulates production of specific IgG antibodies that will no longer recognize the H and N proteins on the virus