Answer:
A. Cell
B. Organ
C. Tissue
Explanation:
cell: The smallest unit of life capable of independent reproduction. Generally contains nucleic acid, cytoplasm, a cell membrane, and many other proteins and structures.
organ: A structure made of different tissues that work together to perform physiological functions.
Tissues: A group of similar cells with the same origin that work together to perform the same function.
A- the ratio to surface area
I think it would be A hope that helps
In the case of breathing, probably because we have gotten so used to it by now.