Why are national defense and street lighting considered public goods?

• A. Because they are costly to provide and therefore must be supplied by the government and paid for through taxes.
• B. Because they are natural monopolies where average costs fall as the scale of provision increases.
• C. Because they generate positive externalities, so private markets would underprovide them unless corrected.
• D. Because they are nonrivalrous and nonexcludable: one person's use does not reduce others' benefits and people cannot be excluded from benefiting.

Answer: (D)