Label the parts $P$,$Q$,$R$,and $S$ in the given diagram of a chordate:

• A
  $P$: Notochord,$Q$: Nerve cord,$R$: Gill slits,$S$: Post-anal tail
• B
  $P$: Nerve cord,$Q$: Notochord,$R$: Gill slits,$S$: Post-anal tail
• C
  $P$: Nerve cord,$Q$: Notochord,$R$: Fins,$S$: Post-anal tail
• D
  $P$: Notochord,$Q$: Nerve cord,$R$: Fins,$S$: Post-anal tail