In which reaction is the underlined substance undergoing reduction?
- A$\text{C}(s) + \text{CO}_2(g) \rightarrow 2\text{CO}(g)$
- B$\underline{\text{Cl}_2}(g) + 2\text{I}^-(aq) \rightarrow \text{I}_2(aq) + 2\text{Cl}^-(aq)$
- C$\underline{\text{Mg}}(s) + \text{CuO}(s) \rightarrow \text{MgO}(s) + \text{Cu}(s)$
- D$\underline{\text{Zn}}(s) + 2\text{H}^+(aq) \rightarrow \text{Zn}^{2+}(aq) + \text{H}_2(g)$