$P$ - Assertion: Some bacteria are Gram-negative.
$Q$ - Reason: Gram-negative bacteria do not retain the Gram stain.

• A
  $P$ and $Q$ are true,and $P$ is the correct explanation of $Q$.
• B
  $P$ and $Q$ are 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.