18. In economics, inferior goods are goods: (a) that are badly made (b) for which the demand falls as income rises (c) for which the demand rises as income rises, but not in the same proportion (d) for which the demand remains constant as income changes 19. As a general rule it always pays a firm to expand output whenever: (a) average revenue (AR) is greater than average cost (AC) (b) marginal revenue (MR)= marginal cost (MC) (c) AR -MC (d) MR is greater than MC 20. If it is observed that in a particular equilibrium market price has fallen and quantity exchanged has increased: (a) it is likely that demand has increased (b) it is likely that supply has decreased (c) it is likely that supply has increased (d) it is likely that demand has decreased 21. If, when income rises by 2%, the quantity sold of the good rises by 12%, the income elasticity of demand is: (a)-1/2 (c) 6 (b) 1/6 (d) -6