Which of the following best describes the mechanisms that contribute to a population achieving herd immunity against an infectious agent?
A. A low proportion of individuals within a population with immunity against the infectious agent.
B. A high proportion of individuals within a population who are susceptible to the infectious agent.
C. A high proportion of individuals within a population who are immune to the infectious agent.
D. A high proportion of individuals within a population with passive-immunity to the infectious agent.