Which compound is a conjugate acid-base pair?
a. H₂PO₄⁻ and HPO₄₂⁻
b. H₃PO₄ and HPO₄₂⁻
c. H₂PO₄⁻ and PO₄₃⁻
d. H₃PO₄ and PO₄₃⁻