Which of the Following Is the BEST Example of Herd

Which of the following is the BEST example of herd immunity?

A) A child infected with measles travels from Germany to the United States.Several babies contract the disease, but the outbreak is largely contained due to vaccinations.
B) Certain populations of cattle are less susceptible to infection with encephalitis because of their genetic makeup.
C) Once a certain threshold of individuals has been infected with a novel human pathogen, it is unlikely that any more will be.
D) Geese and chickens are infected with different strains of influenza because they express different receptors on their cell surface.
E) When infection spreads through a population, certain individuals generate stronger immune responses than others.