Which statement BEST explains why the influenza vaccine must be updated every year?
a
The influenza vaccine does not necessarily stimulate the immune system to produce antigens.
b
In some years, the influenza vaccine kills the virus, while in other years it does not.
c
In some years, the influenza vaccine is not as strong as in others.
d
The influenza virus adapts and mutates every year, so new vaccines must be produced.
d
The influenza virus adapts and mutates every year, so new vaccines must be produced.