Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$: All vertebrates are chordates but all chordates are not vertebrates.
Reason $(R)$: Notochord is replaced by vertebral column in the adult vertebrates.
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
  $(A)$ is correct but $(R)$ is not correct
• C
  $(A)$ is not correct but $(R)$ is correct
• D
  Both $(A)$ and $(R)$ are correct but $(R)$ is not the correct explanation of $(A)$