Of the balanced chemical equations below, which one most correctly represents a combustion reaction?
1.2 N2 + 3 H2 → 2 NH3
2.2 Mg + O2 → 2 MgO
3.2 CH4 + 4 O2 → 2 CO2 + 4 H2O
4.CaO + H2O → Ca (OH)2