22. Describe the four main characteristics of an ani- mal's respiratory surface.

23. Trace the pathway of air through the human res-
piratory system, from the nostrils to the lungs. At
what point within the lungs do the oxygen mol-
ecules really enter the body?

the answer is z.

the human body has five levels of organization. level 1: cells. cells are the basic units of life and form the first level of organization. cells in the human body have specific functions. examples include nerve cells, blood cells, liver cells, and so on.

what? are the statements?

well, since there is no answer choices, i will give an answer on how it works : )

the two objects are placed together. energy will always be transferred from the hotter object to the cooler object until they are equal. hope this is close to one of the answer choices : )