Given below are two statements,one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A:$ Isotopes of hydrogen have almost same chemical properties,but difference in their rates of reaction.
Reason $R:$ Isotopes of hydrogen have different enthalpy of bond dissociation.
In the light of the above statements,choose the most appropriate answer from the options given below:

• A
  Both $A$ and $R$ are correct but $R$ is $NOT$ the correct explanation of $A$
• B
  Both $A$ and $R$ are correct and $R$ is the correct explanation of $A$
• C
  $A$ is not correct but $R$ is correct
• D
  $A$ is correct but $R$ is not correct