$S -$ Assertion: Ostrich is an animal belonging to the class Aves.
$R -$ Reason: Ostrich is a flightless bird.

• 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.