In which equation is the underlined substance functioning as a reducing agent?
- A$3\text{CO} + \text{Fe}_2\text{O}_3 \rightarrow 2\text{Fe} + 3\text{CO}_2$
- B$\text{CO}_2 + \text{C} \rightarrow 2\text{CO}$
- C$\text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O}$
- D$\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2$