An example of a pure public good is

• A. Rival and excludable
• B. Nonrival and nonexcludable
• C. Rival and nonexcludable
• D. Nonrival and excludable

Answer: (B)

A pure public good is nonrival and nonexcludable. That means many people can use it at the same time without diminishing its availability for others, and it’s not feasible to exclude anyone from benefiting. A classic example is national defense: protecting the country doesn’t get used up by one person, and you can’t practically stop someone from being defended.

In contrast, goods that are rival and excludable (like private goods) can be consumed by one person and prevent others from using them, while goods that are rival but nonexcludable (like a congested fishery) become scarcer as more people use them and you can’t easily exclude users. Goods that are nonrival but excludable (like a club or toll road when not congested) can be restricted to paying members, but don’t get used up quickly by one person.

So the combination nonrival and nonexcludable is the hallmark of a pure public good.