7.a/ A public good is indeed non-excludable and non-rival, but it is not necessarily a "free" good. Non-excludability means that once the goods are produced, no one can be prevented from consuming it. Non-rivalry means that one person's consumption of the good does not reduce its availability for others. However, this does not mean that the good is free in the sense of having no cost. Public goods are typically funded by taxes or other public revenue, so there is a cost involved in their provision. The "free" aspect refers to the fact that once the good is provided, additional consumers can enjoy the good at no additional cost as "free riders". As given example from textbook, streetlight is nonexclusionary and nonrival good, everyone can benefit once it becomes available or installed. In this case, the consumers are not directly paying for the cost of producing streetlight, but the costs are funded from consumers 's government tax bill