Which one of the following is a decomposition reaction?
a) 2H 2 (g) + O 2 (g) -> 2H 2O
b) (g) N 2 (g) + 3H 2 (g) -> 2NH 3
c) (g) CH 4 (g) + 2O 2 (g) -> CO 2 (g) + 2H 2O
d) (g) 2H 2O (l) -> 2H 2 (g) + O 2 (g)