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