13. If the income elasticity of demand is 0.80 and quantity demanded increases by 10 percent as a result of a change in income, the income must have a. increased by 8 percent. b. increased by 80 percent. c. decreased by 8 percent. d. decreased by 12.5 percent. 14. When demand is unitary elastic a. marginal revenue is zero. b. the percentage change in quantity equals the percentage change in price. c. an increase in price has no effect on the quantity demanded. d. both a and b e. all of the above 15. When marginal revenue is negative a. MR