When the rate of the reaction 2 NO(g) + O₂(g) → 2 NO₂(g) was studied, the rate was found to double when the O₂ concentration alone was doubled but to quadruple when the NO concentration alone was doubled. Which of the following mechanisms accounts for these observations? Explain your reasoning.

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(I) Step 1 NO + O₂ NO3 and its reverse (both fast, equilibrium) Step 2 NO + NO3 (II) Step 1 NO + NO NO₂ + NO₂ → N₂O₂ Step 2 O₂ + N₂O₂N₂O4 Step 3 N₂O4 NO₂ + NO₂ (slow) (slow) (fast) (fast)