ANSWER:
The correct option is B: Alveoli. Gas exchange of "carbon dioxide and oxygen" takes place in the "alveoli" of the lungs.
Step-by-step explanation:
- Alveoli are present in the alveolar sacs located at the end of respiratory bronchioles. Alveoli have thin walls and look like bubbles in the sacs.
- These also are in "direct contact" with the blood capillaries. This contact ensures that "oxygen" is released into the blood capillaries by the alveoli directly.