$P$: Bats and whales are classified as mammals.
$Q$: The heart of a bat and a whale is four-chambered.

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