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Pail-(s confer man\\,lr external benets on society: open space, trees that reduce pollution, and so on. Therefore, the mail-(et equilibrium quantityr of parks is not equal to the socially optimal quantity. The following graph shows the demand for parks (their private value), the supply of parks (the private cost of producing them), and the social value of parks, including both the private value and external benets. Use the biack point {plus symbol) to indicate the market equilibrium quantity. Next, use the purple point (diamond symbol) to indicate the sociaii'y optimalII quantity. "I- I SUDD'Y (Private Cost) Market Equilibiium '0 Socially Optimal Level PRICE OF PARKS Demand (Private Value)