How do I solve this AP Chemistry problem on Equilibrium?

Question

In which of the following systems would the number of moles of the substances present at equilibrium not be shifted by a change in the pressure of the system at constant temperature?

 

A. CO (g)+NO (g) <==> CO2 (g)+ (1/2)N2 (g)

 

B. N2 (g)+ 2O2 (g) <==> 2NO2 (g)

 

C. N2O4 (g) <==> 2NO2 (g)

 

D. NO (g)+O3 (g) <==>NO2 (g)+ O2 (g)

Answer 1

D. NO (g)+ O3 (g) <==>NO2 (g)+ O2 (g)

 

This is the case since this is the only reaction in which the number of moles of gas does not change from reactant side to product side.  There are 2 moles of gas on each side.