For the reaction 2 SO₂ (g) + O₂ (g) ⇌ 2 SO₃ (g) which of the following is the correct equilibrium-constant expression?

For the reaction which of the following is the correct equilibrium-constant expression?
a. KC = 2[SO₂][O₂]₂[SO₃]
b. KC = [SO₂]₂[O₂][SO₃]₂
c. KC = [SO₃]₂[SO₂]₂[O₂]
d. KC = 2[SO₃]₂[SO₂][O₂]