Which equation represents an oxidation reaction?
- A$\text{C} + \text{O}_2 \rightarrow \text{CO}_2$
- B$\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$
- C$\text{CaO} + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O}$
- D$\text{N}_2\text{O}_4 \rightarrow 2\text{NO}_2$