$S$ - Statement: The myelin sheath around a nerve fiber contains lipids.
$R$ - Reason: This sheath prevents the impulse from leaking into adjacent nerve fibers.

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