Suppose one observes that when the price of peanut butter increases, the demand for jelly increases. One must conclude that:
A.peanut butter and jelly are complements.
B.peanut butter and jelly are inferior goods.
C.peanut butter and jelly are normal goods.
D.peanut butter and jelly are substitutes.
正确答案:peanut butter and jelly are substitutes.