Balance the following equations:
a) PbO2 (s) —> PbO (s) + O2 (g)
b) Fe(OH)3 (s) —> Fe2O3 (s) + H2O (s)
c) (NH4)2CO3 (s) —> NH3 (g) + H2O (g) + CO2 (g)
d) CaCl2 (aq) + H2SO4 (aq) —> CaSO4 (s) + HCl (aq)