What Is the Equilibrium Constant Expression (K_a)for the Acid Dissociation

Multiple Choice

What is the equilibrium constant expression (K_a) for the acid dissociation of hydrocyanic acid HCN? The equation of interest is HCN(aq) + H₂O(l) ⇌ H₃O⁺(aq) + CN^-(aq) .

A) K_a = ([H₃O⁺][CN^-]) /([HCN][H₂O])
B) K_a = ([H₃O⁺][CN^-]) /([HCN])
C) K_a = ([HCN][H₂O]) /([H₃O⁺][CN^-])
D) K_a = ([HCN]) /([H₃O⁺][CN^-])