For the gas-phase reaction NO₂(g) + CO(g) → NO(g) + CO₂(g), two mechanisms are proposed.
Mechanism A:
Step 1 NO₂ + NO₂ → NO₃ + NO
Step 2 NO₃ + CO → NO₂ + CO₂
Mechanism B:
Single step NO₂ + CO → NO + CO₂
During the reaction, NO₃(g) is detected only while the reactants are being consumed and is absent from the final mixture. Explain why this observation supports Mechanism A rather than Mechanism B.