What is the equilibrium expression for the first proton acceptance by a diprotic base (A^2-)?

• A. K_b1 = [HA^-][H₃O⁺]/[A^2-]
• B. K_b1 = [HA^-][OH^-]/[A^2-]
• C. K_b1 = [A^2-][OH^-]/[HA^-]
• D. K_b1 = [H₂A][OH^-]/[HA^-]

Answer: (B)