A. Multiple organ systems working together
Explanation:
All of the organ systems work together to maintain homeostasis in humans because each system does something different. For example, the circulatory system delivers oxygen rich blood to the bones and the bones create new blood cells. When one thing is wrong with one system, it affects all of the systems, like a disease. Another example of this would be the central nervous systems and endocrine working together to coordinate messages to the brain about the body's state while resting and exercising to maintain homeostasis.
The outer skeleton provides protection due to the fact that invertebrates do not have backbones
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