Statements about heavy water are given below.
A. Heavy water is used in exchange reactions for the study of reaction mechanisms.
B. Heavy water is prepared by exhaustive electrolysis of water.
C. Heavy water has higher boiling point than ordinary water.
D. Viscosity of $$H_2O$$ is greater than $$D_2O$$.
Choose the most appropriate answer from the options given below:

Let us analyze each statement about heavy water ($$D_2O$$):

Statement A: Heavy water is used in exchange reactions for the study of reaction mechanisms. This is true. $$D_2O$$ is widely used as a tracer in studying reaction mechanisms through H-D exchange reactions.

Statement B: Heavy water is prepared by exhaustive electrolysis of water. This is true. During electrolysis, $$H_2O$$ is decomposed preferentially over $$D_2O$$ because the O-H bond breaks more easily than the O-D bond. Repeated (exhaustive) electrolysis concentrates $$D_2O$$.

Statement C: Heavy water has a higher boiling point than ordinary water. This is true. $$D_2O$$ has a boiling point of 101.4°C compared to 100°C for $$H_2O$$, due to stronger hydrogen bonds involving deuterium.

Statement D: Viscosity of $$H_2O$$ is greater than $$D_2O$$. This is false. $$D_2O$$ has a higher viscosity than $$H_2O$$ because of stronger intermolecular interactions due to the greater mass of deuterium.

Therefore, statements A, B, and C are correct, and the answer is **A, B and C only**.