Classify each of the following reactions into one of the four general categories: combination, deomposition, single displacement, or double displacement
a. H2SO4(aq) + 2 KOH(aq) --> K2SO4(aq) + 2 H2O(l)
b. 2 S(s) + 3 O2(g) --> 2 SO3(g)
c. NH4NO2(s) --> N2(g) + 2 H2O(g)
d. 2 Al(s) + 6 HCl(aq) --> 2AlCl3(aq) + 3 H2(g)