28. According to the Michaelis-Menten equation, Vo = [S] (Vmax) / ([S] + Km), a high Km value for an enzyme-substrate pair indicates: (A) The enzyme has a high specificity for the substrate. (B) The enzyme works at maximum rate even at low substrate concentration. (C) The enzyme is saturated at very low substrate concentration. (D) The enzyme has a low affinity for the substrate.