Final answer:
When a good is free, people will continue to increase consumption until marginal utility is equal to zero.
Step-by-step explanation:
A rational consumer would only purchase additional units of a good as long as the marginal utility exceeds the opportunity cost. Marginal utility refers to the additional satisfaction or benefit gained from consuming one more unit of a good. The opportunity cost is the value of the next best alternative foregone in order to consume a particular good.
When a good is free, the opportunity cost is zero. Therefore, people will continue to increase consumption until marginal utility is equal to zero. This means that the additional satisfaction gained from consuming one more unit of the free good will become negligible, and it is no longer worth it for the consumer to consume more.
For example, if a restaurant offers free refills on soda, a consumer may continue to drink more soda until the satisfaction they gain from consuming one more refill is minimal.