(4) imaginary number Explanation: In economics, the cross elasticity of demand or cross-price elasticity of demand measures the responsiveness of the demand for a good to a change in the price of another good. It is measured as the percentage change in demand for the first good that occurs in response to a percentage change in price of the second good. A negative cross elasticity denotes two products that are complements, while a positive cross elasticity denotes two substitute products.