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 $(R):$ The members of subphylum Vertebrata possess a notochord during the embryonic period; the notochord is replaced by a cartilaginous or bony vertebral column in adults.
In the light of the above statements,choose the correct answer from the options given below:

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