Which of the following reactants is the acid?

Which is acting as the base?

A. HNO₃ + H₂O ➞ H₃O⁺¹ + OH^-1

B. HCO₃^-1 + H₂PO₄^-1 ➞ H₂CO₃ + HPO₄^-2

C. NH₃ + H₂O ➞ NH₄^-1 + OH^-1