Which of the following might lead to an increase in the equilibrium price of jelly and a decrease in the equilibrium quantity of jelly sold?
a) an increase in the price of peanut butter, a complement to jelly
b) an increase in the price of Marshmallow Fluff, a substitute for jelly
c) an increase in the price of grapes, an input to jelly
d) an increase in consumers’ incomes, as long as jelly is a normal good