The figure shows a diagrammatic view of the human respiratory system with labels $A, B, C$,and $D$. Select the option which gives the correct identification and main function and/or characteristic.

• A
  $C$ - Alveoli - Thin-walled vascular bag-like structures for the exchange of gases.
• B
  $D$ - Lower end of lungs - Diaphragm pulls it down during inspiration.
• C
  $A$ - Trachea - Long tube supported by complete cartilaginous rings for conducting inspired air.
• D
  $B$ - Pleural membrane - Surrounds ribs on both sides to provide a cushion against rubbing.