Identify the substance, from the system listed below, that behaves one time as an acid and another time as a base:
HSO₃- (aq) + HSO₃- (aq) -> H₂CO₃ (aq) + SO₂³-(aq)
HSO₃- (aq) + PO₃⁴- (aq) -> H₂PO₂⁴- (aq) + SO₂- (aq)
HCO₃- (aq) + HS- (aq) -> CO₂³- (aq) + H₂S- (aq)
a. HCO₃ (aq)
b. HPO₂ (aq)
c. HSO₃ (ag)
d. HS (aq)