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