Which statement about non-excludable goods best captures why non-excludability is not sufficient for a public good?

• A. Non-excludability alone guarantees a public good.
• B. Non-excludability always means non-rivalry.
• C. Non-excludability implies that markets will always fail to provide the good.
• D. Non-excludability plus non-rivalry is required; some non-excludable goods may be congestible or rival.

Answer: (D)

Non-excludability means you can’t easily prevent people from using the good, but a true public good also must be non-rival in consumption—the act of one person using it doesn’t reduce another’s ability to use it. If a good is non-excludable yet rival when crowded (a congestible or common-resource good), additional users diminish the value for everyone, so it isn’t a pure public good. Therefore, non-excludability alone does not guarantee a public good; you need both non-excludability and non-rivalry. Some non-excludable goods are congestible or rival, which is why the best answer emphasizes that non-excludability plus non-rivalry is required.