Which one of the following is a decomposition reaction?

A. 2H2O (l) → 2H2 (g) + O2 (g)
B. N2 (g) + 3H2 (g) → 2NH3 (g)
C. CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)
D. 2H2 (g) + O2 (g) → 2H2O (g)