Assertion $(A)$: $Ascidia$ and $Salpa$ are placed under sub-phylum $Urochordata$.
Reason $(R)$: In $Ascidia$ and $Salpa$,notochord is present only in larval tail.
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
  $(A)$ is correct but $(R)$ is not correct
• C
  $(A)$ is not correct but $(R)$ is correct
• D
  Both $(A)$ and $(R)$ are correct and $(R)$ is the correct explanation of $(A)$