12. Enzyme X exhibits maximum activity at pH = 6.9. X shows a fairly sharp decrease in its activity when the pH goes much lower than 6.4. One likely interpretation of this pH activity is that: (A) a Glu residue on the enzyme is involved in the reaction. (B) a His residue on the enzyme is involved in the reaction. (C) the enzyme has a metallic cofactor. (D) the enzyme is found in gastric secretions. (E) the reaction relies on specific acid-base catalysis.