Which of the following correctly describes the equilibrium expression for the first dissociation of a diprotic acid (H₂A)?

• A. K_a1 = [A^2-][H₃O⁺]/[HA^-]
• B. K_a1 = [HA^-][H₃O⁺]/[H₂A]
• C. K_a1 = [H₂A][H₃O⁺]/[HA^-]
• D. K_a1 = [H₂A][OH^-]/[HA^-]

Answer: (B)