Choose the exothermic reaction that is used to ignite the Mg + Oxygen reaction (step 2) that eventually lights the thermite.

2 H2 + O2 ----> 2 H2O
2 Na + O2 ----> Na2O
CH4 + 2 O2 ----> 2 H2O + CO2
2 C4H10 + 13 O2 ----> 8 CO2 + 5 H2O