Answer
a) A decrease in the price of flour.
b) An increase in the price of milk.
Work Step by Step
a) Please see the upper graph. Both the quantity of cream cheese and the quantity of bagels increased. Since cream cheese is a complementary good, the decrease in the price of flour increased demand for bagels. Increased demand for bagels increases demand for cream cheese.
b) Please see the lower graph. The number of bagels decreased but the price of cream cheese increased. Since the price of cream cheese increased, the price of milk increased. (Milk is an input for cream cheese.) Since bagels and cream cheese are complements, the increase in the price of cream cheese would decrease the equilibrium quantity of bagels.
