Of the reactions below, which one is a decomposition reaction?
A) 2Mg + O2 → 2MgO
B) 2CH4 + 402 → 2CO2 + 4H2O
C) 2N2 + 3H2 → 2NH3
D) Cd(NO3)2 + Na2S → CdS + 2NaNO3
E) NH4CI → NH3 + HCI