In which equation does the underlined substance undergo oxidation?
- A$2\text{CuO} + \underline{\text{C}} \rightarrow \text{CO}_2 + 2\text{Cu}$
- B$\text{Fe}_2\text{O}_3 + 3\underline{\text{CO}} \rightarrow 2\text{Fe} + 3\text{CO}_2$
- C$2\text{Mg} + \text{O}_2 \rightarrow \underline{2\text{MgO}}$
- D$\underline{\text{MnO}_2} + 4\text{HCl} \rightarrow \text{MnCl}_2 + 2\text{H}_2\text{O} + \text{Cl}_2$