Why are secondary or ʺboosterʺ reimmunizations given?

A) Secondary immunizations are necessary for an innate immune response.
B) Secondary reimmunizations produce a secondary immune response and boost antibody titers.
C) Secondary reimmunizations produce a phagocytic immune response that is longer lasting than the primary immune response.
D) Frequent secondary reimmunizations are needed because the immune system doesnʹt remember antigens for more than a few years.