a. The increase in quantity demanded from 20 to 30 is due to a rightward shift of the demand curve arising from an increase in demand. b. D1 to D2 represents a shift of the demand curve c. The change in quantity demanded (from 20 to 30) is due to a change in price. d. A decrease in income can cause the demand curve to shift rightwards. e. An increase income can cause the demand curve to shift rightwards. f. If buyers expect price of a good to fall, they will buy the good immediately