37. One enzyme, M protein, exhibits maximum activity at pH = 6.9. M protein 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) the enzyme is a calcium-binding protein (B) the enzyme is found in saliva. (C) a Asp residue on the enzyme is involved in the reaction. (D) a His residue on the enzyme is involved in the reaction.