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11 sense: the surface of the Earth is limited, 12 absolute scarcity, but the scarcity that concerns economists is the relative scarcity of resources in 13 uses. Materials used for one purpose cannot at the same time be used for other purposes; if the quantity of an input is limited, the increased use of it in one manufacturing process must cause it to become less available for other uses. The cost of a product in 14 of money may not measure its true cost to society. The true cost of, say, the construction of a supersonic jet is the value of the schools and refrigerators that will never be built as a result. Every act of production uses up some of societys available resources; it means the foregoing of an opportunity to produce something else. In deciding how to use resources most effectively to 15 the wants of the community, this opportunity cost must 16 be taken into account. In a market 17 the price of a commodity and the quantity supplied depend on the cost of making it, and that cost, ultimately, is the cost of not making other goods. The market mechanism enforces this 18 . The cost of, say, a pair of shoes is the price of the leather, the labor, the fuel, and other elements used up in producing them. But the price of these inputs, in turn, 19 on what they can produce elsewhereif the leather can be used to produce handbags that are valued highly by consumers, the price of leather will be bid up 20 | 
