Consider two goods, plane tickets and milk. Suppose Sharon is to choose the optimal quantities of both goods. Use the following selections to identify the utility-maximizing rule Sharon should follow to choose the optimal quantities. (Note: In the answer options that follow, MU stands for "marginal utility.")

Since milk costs less than plane tickets, it must follow that when Sharon chooses optimal quantities of milk and plane tickets, the marginal utility received from the last plane ticket isthan the marginal utility from the last gallon of milk.