The price elasticity of demand is a​ units-free measure of the responsiveness of​ ______. A. the demand for a good when the price of one of it substitutes or a complement of it changes B. the quantity demanded of a good to a change in the quantity supplied when all other influences on buying plans remain the same C. the demand for a good when​ consumers' income changes D. the quantity demanded of a good to a change in its price when all other influences on buying plans remain the same