$A$: The most significant contribution of mitosis is cell repair.
$R$: Mitosis is essential for the continuous replacement of cells in the outermost layer of the epidermis,the lining of the gut,and blood cells.

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