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.

Inventory control is modeled by the Markovian type of this concept, and in the Keynesian Cross, this equals the total output of an economy at efficiency. The Slutsky equation relates two forms of this. The Hicksian form of it assumes constant utility and is usually contrasted with the Marshallian form. Say's Law states that this is created by the presence of goods, and Keynes did work on the "aggregate" form of it. This quantity has a downward sloping curve and can be defined as the amount of a good or service consumers are willing to buy at a given price. For 10 points, identify this economic concept, commonly contrasted with supply.
Science