Which of the following expressions is the correct equilibrium-constant expression for the equilibrium between dinitrogen tetroxide and nitrogen dioxide?

N2O4(g) ⇌ 2NO2(g)

a. [NO]^2 [N2O4]
b. [NO2]/ [N2O4]^2
c. [NO2]/[N2O4]
d. [NO2]^2/[N2O4]