The figure shows a diagrammatic view of human respiratory system with labels A, B, C and D. Select the option which gives correct identification and main function and/or characteristic.
D - Lower end of lungs - diaphragm pulls it down during inspiration.
A - trachea - long tube supported by complete cartilaginous rings for conducting inspired air.
B - pleural membrane - surround ribs on both sides to provide cushion against rubbing.
C - Alveoli - thin walled vascular bag like structures for exchange of gases.