Equilibrium quantity must decrease when demand a. decreases and supply does not change, when demand does not change and supply increases, and when both demand and supply decrease. b. decreases and supply does not change, when demand does not change and supply decreases, and when both demand and supply decrease. c. increases and supply does not change, when demand does not change and supply increases, and when both demand and supply decrease. d. increases and supply does not change, when demand does not change and supply decreases, and when both demand and supply decrease.