For the heterogeneous equilibrium
CaCO3(s) ⇌ CaO(s) + CO2(g)
a sealed vessel is at equilibrium at a fixed temperature when additional powdered CaCO3 is introduced. Which change occurs as the system returns to equilibrium?
a) The equilibrium shifts to the right, increasing PCO2.
b) The equilibrium shifts to the left, decreasing PCO2.
c) There is no shift because the activities of pure solids remain constant.
d) The value of K decreases because more solid reactant is present.