2: Supply and Demand
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Thumbnail: The price P of a product is determined by a balance between production at each price (supply S) and the desires of those with purchasing power at each price (demand D). The diagram shows a positive shift in demand from D1 to D2, resulting in an increase in price (P) and quantity sold (Q) of the product. (CC BY-SA 3.0; Paweł Zdziarski via Wikipedia)