$P$ (Assertion): The first circulatory system is observed in the phylum Annelida.
$Q$ (Reason): The first true coelom is observed in the phylum Annelida.

• A
  $P$ and $Q$ are both true,and $P$ is the correct explanation of $Q$.
• B
  $P$ and $Q$ are both true,but $P$ is not the correct explanation of $Q$.
• C
  $P$ is true,but $Q$ is false.
• D
  $P$ is false,but $Q$ is true.