What is the equilibrium expression for a weak acid HA in water?

• A. [HA][H₂O]/[A^-]
• B. [HA][A^-]/[H₃O⁺]
• C. [A^-][H₂O]/[H₃O⁺]
• D. [A^-][H₃O⁺]/[HA]

Answer: (D)