Which of the following events would unambiguously cause an increase in the equilibrium price of cotton shirts?

a. an increase in the price of wool shirts and a decrease in the price of raw cotton
b. a decrease in the price of wool shirts and a decrease in the price of raw cotton
c. an increase in the price of wool shirts and an increase in the price of raw cotton
d. a decrease in the price of wool shirts and an increase in the price of raw cotton

c