A perfectly competitive market is a market where there are many buyers and sellers of identical goods. The price of a good is determined by market forces. This means that price is determined at the intersection of the demand curve and supply curve for a good.
If a seller attempts to set the price for his good, the demand for his good will fall to zero as consumers would patronise other sellers who sell identical goods at a cheaper price. This means that the demand for goods in a perfectly competitive firm is perfectly elastic. Thus, the demand curve is horizontal.
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