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.

1. A. A - trachea - long tube supported by complete cartilaginous rings for conducting inspired air
2. B. B-pleural membrane - surround ribs on both sides to provide cushion against rubbing
3. C. C-Alveoli - thin walled vascular bag like structures for exchange of gases
4. D. D-Lower end of lungs - diaphragm pulls it down during inspiration

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