What stress would shift the equilibrium position of the following system to the right?

N2O3(g) ⇌ NO(g) + NO2(g); ΔH is positive

Decreasing the concentration of N2O3
Heating the system
Adding a catalyst
Increasing the concentration of NO

Heating the system

Explanation:

It's the correct answer on the test. I don't understand this very well, but adding a catalyst doesn't change the equilibrium, and increasing the concentration of NO would shift it to the left. Heating the system will shift it right in an endothermic equation, which this is.