Which reaction has an enthalpy change that is an enthalpy change of formation?
- A$\text{C}(g) + 2\text{H}_2(g) \rightarrow \text{CH}_4(g)$
- B$\tfrac{1}{2}\text{N}_2(g) + \tfrac{1}{2}\text{O}_2(g) \rightarrow \text{NO}(g)$
- C$\text{Na}_2\text{O}(s) + \text{SO}_3(g) \rightarrow \text{Na}_2\text{SO}_4(s)$
- D$\text{PCl}_3(g) + \text{Cl}_2(g) \rightarrow \text{PCl}_5(g)$