For which of the following reaction, the unit of equilibrium constant (Kc) is reciprocal of molar concentration (M-1):

H2(g)+I2(g)⇌N2O4

2NO2(g)⇌N2O4(g)

3H2(g)⇌2H3N(g)

H2(g)+O2(g)⇌2NOg