Getting a Flu Shot Reduces the Chances of Spreading the Illness

Getting a flu shot reduces the chances of spreading the illness to one's classmates and friends. Why, then, is this considered a market failure due to external benefits?

A) because the number of people who obtain flu shots is less than the socially optimal quantity
B) because too many people actually get the flu shot
C) because flu shots are not 100% effective
D) because flu shots are priced too high relative to the cost of producing them