Which of the following is a combustion reaction?

A. C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (g)
B. 2SO3 (g) → 2SO2 (g) + O2 (g)
C. 2Na (s) + Cl2 (g) → 2NaCl (s)
D. 2KClO3 (s) → 2KCl (s) + 3O3 (g)