$S$ - Statement: The cytoskeleton is composed of three types of filaments: microfilaments, microtubules, and intermediate filaments.
$R$ - Reason: Microtubules are hollow tubes made of the globular protein tubulin.

• 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.