Which equation does not represent a redox reaction?
a) Mg + 2HCl → MgCl₂ + H₂
b) CH₄ + 2O₂ → CO₂ + 2H₂O
c) Fe + CuSO₄ → FeSO₄ + Cu
d) CuO + 2HCl → CuCl₂ + H₂O