Use the model of supply and demand to explain how the fall in the price of frozen yogurt would affect the price of ice-cream and the quantity of ice-cream sold. In your explanation, write a demand function for ice-cream and a supply function for ice-cream and identify the exogenous and the endogenous variables. Then draw a graph of the ice-cream market in equilibrium and show how the demand or the supply curve shift due to the fall in the price of frozen yogurt. Indicate the new equilibrium price and quantity.