Consider this reaction, carried out at constant volume.
2 SO2 (g) + O2 (g) ⇌ 2 SO3 (g) Δ H = -198kJ
The concentration of O2 (g) at equilibrium increases if
a) SO2 is added to the system.
b) SO3 is added to the system.
c) the temperature of the system is lowered.
d) an inert gas is added to the system.