What are the correct formulas of the following ions?
a. Ammonium-(a)NH4^+(b)NH4^+(c)NH3^+(d)NH4^+
b. Hydrogencarbonate-(a)HCO3^-(b)HCO3^-2(c)HCO3^-(d)HCO3^-2
c. Sulfate-(a)SO4^-2(b)SO4^-2(c)SO4^-3(d)SO4^-