In which reaction does the species shown in bold act as an oxidising agent?
- A$\mathbf{2Ca}$ + \text{O}_2 \rightarrow 2CaO$
- B$\text{Cr}_2\text{O}_7^{2-} + 8\text{H}^+ + \mathbf{3SO}_3^{2-} \rightarrow 2\text{Cr}^{3+} + 4\text{H}_2\text{O} + 3\text{SO}_4^{2-}$
- C$\text{Mg} + \mathbf{Fe}^{2+} \rightarrow \text{Mg}^{2+} + Fe$
- D$\mathbf{SO}_2 + 2\text{H}_2\text{O} + 2\text{Cu}^{2+} + 2\text{Cl}^- \rightarrow \text{H}_2\text{SO}_4 + 2\text{H}^+ + 2\text{CuCl}$