The Mother Asks the Nurse Why Her Child Must Have

The mother asks the nurse why her child must have three doses of a certain vaccine over the next few months.Which response by the nurse is correct?

A) "For some diseases,multiple doses are needed over time to allow the development of full immunity."
B) "The drug level must be high enough to prevent disease."
C) "It is prophylactic because not all parents get the immunizations at the correct time."
D) "We are not certain,but more doses have proven to be more effective."