Which of the following is a non-excludable, non-rival public good?

• A. National defense
• B. A private gym
• C. A toll road
• D. A movie ticket

Answer: (A)

Public goods are non-excludable and non-rival: once provided, you can’t realistically exclude people from benefiting, and one person’s use doesn’t reduce another’s ability to enjoy it. National defense fits this because protection is available to all residents within the country, and protecting one person doesn’t cut into protection for others. In contrast, a private gym is excludable (you must pay or be a member) and rival (limited space means one person’s use can limit another’s). A toll road is excludable (you must pay to use it) and can be congested, making it rival in some cases. A movie ticket is excludable (requires a ticket) and rival (limited seats). Thus, national defense is the non-excludable, non-rival public good.