Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: Abingdon tortoise in Galapagos islands became extinct within a decade after goats were introduced.
Reason $R$: Goats were more efficient at browsing than Abingdon tortoise.
In the light of the above statements, choose the most appropriate answer from the options given below:

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