For Many Enzymes, the Rate of Product Formation Increases as the Concentration

For many enzymes, the rate of product formation increases as the concentration of the substrate increases. Eventually the rate of the reaction reaches a maximum at which further increases in the concentration of the substrate have no effect. Why?

A) At high concentrations of substrate, the activation energy of the reaction decreases.
B) The enzyme is no longer specific for the substrate.
C) At high concentrations of substrate, the reaction is endergonic.
D) At high concentrations of substrate, all of the enzyme active sites are being occupied by substrate molecules.
E) At high concentrations of substrate, the activation energy of the reaction increases.