Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R).$
Assertion $(A):$ Heavy water is used for the study of reaction mechanism.
Reason $(R):$ The rate of reaction for the cleavage of $O-H$ bond is slower than that of $O-D$ bond.
Choose the most appropriate answer from the options given below:

• A
  Both $(A)$ and $(R)$ are true but $(R)$ is not the true explanation of $(A).$
• B
  Both $(A)$ and $(R)$ are true and $(R)$ is the true explanation of $(A).$
• C
  $(A)$ is false but $(R)$ is true.
• D
  $(A)$ is true but $(R)$ is false.