Which of the Following Is a Real-World Example of a Public

Which of the following is a real-world example of a public good?

A) You are craving a Diet Coke® for lunch, so you purchase one from the hot dog stand in front of your classroom building.
B) National defense is available even for people who do not pay income taxes.
C) A person seated in a non-smoking section of a restaurant decides to sneak a few puffs of a cigarette.
D) A child receives a vaccination to prevent the spread of measles.