Given the task definition and input, reply with output. In this task you will be given a text passage from a trivia quiz. You should choose the category this question belongs to from these categories: History, Science, Social Science, Fine Arts, Literature. You can only choose one category, so if in doubt, please choose the closest match.

To maximize profit, firms should advertise until the ratio of advertising costs to sales revenue equals the ratio of two measures of elasticity of this concept. One variety of this concept is plotted on the y-axis of a Keynesian cross, and the cross-price elasticity of it is the change in it for one good when another's price changes. As income increases, this quantity for (*) normal goods also increases, and Giffen goods are exceptions to the law of this quantity, which states that it goes down when price increases. For ten points, identify this quantity from economics, which is negatively related to supply.
Science