$$2 NO_2(g) \rightleftharpoons 2 NO(g) + O_2(g)$$ The above materials were sealed in a flask and allowed to come to equilibrium at a certain temperature. A small quantity of $O_2(g)$ was added to the flask, and the mixture was allowed to return to equilibrium at the same temperature. Which of the following has increased over its original equilibrium value?  

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