What are the two characteristics that differentiate private goods from public goods?

The two characteristics that differentiate private goods from public goods are rivalry and excludability. Rivalry means that the consumption of the good by one person reduces the amount available for others to consume. Excludability means that it is possible to prevent someone from consuming the good if they do not pay for it. Private goods are both rivalrous and excludable, while public goods are non-rivalrous and non-excludable. Negative and positive externalities refer to the impact of a good on third parties, while marginal cost and marginal benefit refer to the additional cost or benefit of producing or consuming one more unit of a good. Ownership and usage are not characteristics that differentiate private goods from public goods. Therefore, the correct answer is A.