$S$ - Statement: $RNA$ contains thymine in its structure.
$R$ - Reason: $DNA$ does not contain thymine in its 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
  $S$ and $R$ are both false.