Which statement about public goods is correct?

Which statement about public goods is correct?

• A. A public good is excludable and rivalrous; markets provide them efficiently; example private roads.
• B. A public good is non-excludable and non-rivalrous; markets underprovide; example national defence.
• C. A public good is non-excludable and rivalrous; markets provide them efficiently.
• D. A public good is non-excludable and non-rivalrous; government provision improves efficiency.

Answer: (D)