Describe what happens in the alveoli.

Gas exchange

Explanation:

Gas exchange, since oxygen is diffused into the blood through the alveoli and into the capillaries. This is to oxygenate any deoxygenated blood, of which carbon dioxide is diffused out into the alveoli and we breathe this out.

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.