Which of the following is the correct equilibrium constant expression for the reaction:

2H₂O(g) + 2Cl₂(g) ↔ 4HCl(g) + O₂(g)

a. Keq = [H₂O][Cl₂] / [HCl][O₂]
b. Keq = [HCl][O₂] / [Cl₂][H₂O]
c. Keq = [HCl]₄[O₂] / [H2O]₂ [Cl₂]₂
d. Keq = [H2O]₂[Cl₂]₂ / [HCl]₄[O₂]