Its the second option....................................... :)
Corrosion: Rain and acid rain
<span>Breaking due to freezing and thawing: Temperature
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Answer:
The alveoli are sacks coated on their inner wall by white and sticky liquid, they can be more than a millimeter in diameter and surface active agent. In them there is the exchange of gases between O2 and CO2.
They are evaginations of the epithelium of the air ducts with a single opening for the gases to exit and enter, controlled by the action of a smooth muscle sphincter. Its walls, called alveolar septa, provide a large increase in the exchange surface.
The innermost or farthest is the Hypodermis