Public Schools Require Proof That a Child Has Been Immunized

Multiple Choice

Public schools require proof that a child has been immunized against several diseases before enrolling. Why?

A) People don't realize that becoming vaccinated entails no risk.
B) It is a costly to fake signal that a child is healthy.
C) It is an efficient way to publicize the need for immunizations.
D) People do not adequately account for the external cost of not vaccinating their children.