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