In the Presence of Alcohol Dehydrogenase, the Rate of Reduction

In the presence of alcohol dehydrogenase, the rate of reduction of acetaldehyde to ethanol increases as the concentration of acetaldehyde is increased. Eventually, the rate of the reaction reaches a maximum, at which point further increases in the concentration of acetaldehyde have no effect. Why?

A) All the alcohol dehydrogenase molecules are bound to acetaldehyde molecules.
B) At high concentrations of acetaldehyde, the activation energy of the reaction increases.
C) At high concentrations of acetaldehyde, the activation energy of the reaction decreases.
D) The enzyme is no longer specific for acetaldehyde.
E) At high concentrations of acetaldehyde, the change in free energy of the reaction decreases.