If the price of walnuts rises, many people would switch from consuming walnuts to consuming pecans. but if the price of salt rises, people would have difficulty purchasing something to use in its place. these examples illustrate the importance of a. the availability of close substitutes in determining the price elasticity of demand. b. a necessity versus a luxury in determining the price elasticity of demand. c. the definition of a market in determining the price elasticity of demand. d. the time horizon in determining the price elasticity of demand.