$P$ - Statement: Birds and mammals have a four-chambered heart.
$Q$ - Reason: In mammals,the aortic arch curves to the left.

• 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 and $Q$ is false.
• D
  $P$ is false and $Q$ is true.