When some of the costs of a good spill over to a third party: Select an answer and submit. For keyboard navigation, use the up/down arrow keys to select an answer. a a negative externality exists and the good tends to be overproduced by private markets. a negative externality exists and the good tends to be underproduced by private markets. C a positive externality exists and the goods tends to be underproduced by private markets. d a positive externality exists and the goods tends to be overproduced by private markets.