The market structure affects price decisions. For example, under pure competition, no single seller can affect the market price. On the other hand, a seller can set the price it wants under pure monopoly because there is only one seller in the market. Demand-price relationship should be well-understood before determining the price of a product. Normally, there is a negative relationship between price and demand. In other words, when the price increases, the demand decreases. Price elasticity of demand refers to reaction of demand to a price change. D is the right asnwer.