Awkwardly worded question but I'll try...
First off, we can eliminate Alveoli and Blood Cells. These are key in the cardiovascular system, but not in other systems of the body.
I'm inclined to say choice A. since the structure of an anatomical system is really what defines what the system does. For instance, the physical structure of skin cells prevents many pathogens from penetrating it and the existence of a highly active layer of skin cells (basal layer) is the reason our skin doesn't get worn down with time.
