Refer to the above diagram, in which S1 and D1 represent the original supply and demand curves and S2 and D2 the new curves. In this market the indicated shift in demand may have been caused by:

A. an increase in incomes if the product is a normal good.
B. an increase in incomes if the product is an inferior good.
C. a decline in the number of buyers in the market.
D. a decline in the price of a substitute good.

ANSWER:

A. an increase in incomes if the product is a normal good.