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 $\mathrm{O}-\mathrm{H}$ bond is slower than that of $\mathrm{O}-\mathrm{D}$ bond.
Choose the most appropriate answer from the options given below :

• A

  Both $\left(\mathrm{A}\right)$ and $\left(\mathrm{R}\right)$ are true and $\left(\mathrm{R}\right)$ is the true explanation of $\left(\mathrm{A}\right)$.
• Correct answer
  B

  $\left(\mathrm{A}\right)$ is true but $\left(\mathrm{R}\right)$ is false.
• C

  $\left(\mathrm{A}\right)$ is false but $\left(\mathrm{R}\right)$ is true.
• D

  Both $\left(\mathrm{A}\right)$ and $\left(\mathrm{R}\right)$ are true but $\left(\mathrm{R}\right)$ is not the true explanation of $\left(\mathrm{A}\right).$

Hence, the correct answer is option: 'B'.