19. For the equilibrium that exists in an aqueous solution of nitrous acid $(\mathrm{HNO}_2$ , a weak acid), the equilibrium constant expression is:
(A) \(K = \frac{[\mathrm{H}^{+}][\mathrm{NO}_{2}^{-}]}{[\mathrm{HNO}_{2}]}\)
(B) \(K = \frac{[\mathrm{H}^{+}][\mathrm{N}^{2+}][\mathrm{O}^{2-}]^2}{[\mathrm{HNO}_2]}\)
(C) \(K = [\mathrm{H}^{+}]\mathrm{[NO}_{2}^{-}]\)
(D) \(K = \frac{[\mathrm{H}^{+}]^{2}[\mathrm{NO}_{2}^{-}]}{[\mathrm{HNO}_{2}]}\)