$S -$ Statement: Based on the position of the centromere,chromosomes are classified into four types.
$R -$ Reason: Chromosomes contain a centromere,which is a primary constriction or disc-shaped structure.

• 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.