When the aldol reaction of acetaldehyde is carried out in D2O, no deuterium is found in the

Question:

When the aldol reaction of acetaldehyde is carried out in D2O, no deuterium is found in the methyl group of unreacted aldehyde. However, in the aldol reaction of acetone, deuterium is incorporated in the methyl group of the unreacted acetone. Explain this difference in behavior.