[单选题]
Which expression reprs) sg0/ y uak(u0eydj+a4v;,qe,djcc x tw 2:kesents $K_a$ for $CH_3COOH$?
A. $\frac{[CH_3COOH^-][H^+]}{[CH_3COOH]}$
B. $\frac{[CH_3COOH]}{[CH_3COOH^-][H^+]}$
C. $\frac{[CH_3COOH^-]}{[CH_3COOH][OH^-]}$
D. $\frac{[CH_3COOH][OH^-]}{[CH_3COOH^-]}$