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 :

Options

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