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.
(a) A-trachea-long tube supported by complete cartilaginous rings for conducting inspired air
(b) 8-pleural membrane-surround ribs on both sides to provide cushion against rubbing
(c) C-alveoli-thin walled vascular bag-like structures for exchange of gases
(d) D-lower end of lungs-diaphragm pulls it down during inspiration