Explain how simultaneous shifts in demand and supply curves could explain these situations:
a. The price of insulin injection kits, used by diabetic patients, increases from $45 to $52, but the equilibrium quantity remains the same.
b. A pest attack on the tomato crop increases the cost of producing ketchup. A mild winter causes cattle herds to be unusually large, causing the price of hamburgers to fall. The equilibrium quantity of ketchup is unchanged.