Write the expression for the equilibrium constant Keq for the reaction below:

2CO(g) + O2(g) ⇔ 2CO2(g)

a. [CO2] / [CO] [O2]

b. [CO]2 / [CO2]2 [O2]

c. [CO2]2 / [CO]2 [O2]

d. [CO]2 [O2] / [CO2]2