Use the Law of Diminishing Marginal Utility to discuss why Market Demand has to have a downward slope. In other words, in order for consumers to demand a higher quantity of an item, its price must fall.

Also use the Law of Diminishing Marginal Utility to explain why consumers often prefer to purchase a variety of goods and services (in other words, we are unlikely to fill a grocery cart with many units of the exact same product).

Finally, use the utility optimizing condition that the ratio of Marginal Utility to Price for any good has to be the same across all goods to justify why it may be rational behavior to buy an item that is expensive compared to other items.