$S -$ Statement: The efficient size of the cell is maintained by meiosis.
$R -$ Reason: The specific type and fixed number of chromosomes are maintained in organisms generation after generation by mitosis.

• 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
  $S$ and $R$ are both false.