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

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

Answer: (A)