The free rider problem occurs because: a. consumption is rivalrous, so the consumption of a product by one individual diminishes the amount available for others. b. individuals are not required to pay for those goods which do not yield any utility to them. c. it is easy to exclude others from consuming a good. d. external costs are imposed on others not directly involved in the transaction. e. exclusion is costly or impossible, so a consumer or producer can use a good without having to pay for it.