This Graph Illustrates the Marginal Cost,marginal Private Benefit,and Marginal Social

This graph illustrates the marginal cost,marginal private benefit,and marginal social benefit of immunization against a contagious childhood disease.
As more children are immunized,the marginal cost increases because ____ and the marginal benefit decreases because ___.

A) of the low-hanging-fruit principle;it is a prisoner's dilemma.
B) of the low-hanging-fruit principle;the probability of spreading the disease declines.
C) the probability of spreading the disease declines;of the low-hanging-fruit principle.
D) of diminishing marginal utility;the probability of spreading the disease declines.