Yearly Vaccination of Humans for Influenza Viruses Is Necessary Because

Yearly vaccination of humans for influenza viruses is necessary because

A) of an increase in immunodeficiency diseases.
B) flu can generate anaphylactic shock.
C) surviving the flu one year exhausts the immune system to non-responsiveness the second year.
D) mutations of DNA in rapidly evolving generations of flu viruses result in altered surface proteins in infected host cells.
E) flu leads to autoimmune disorders.