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A
The structure of body parts
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B
How the structures of the body function
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C
The history of medicine
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D
Chemical reactions only
Why this is the answer
Physiology is the study of how the structures of the body function — how organs, tissues, and systems work and interact. It is paired with anatomy, which is the study of body structure. For example, anatomy describes the structure of a sweat gland, while physiology explains how it produces sweat to help regulate temperature. Cosmetologists apply basic physiology when they understand how skin, hair, nails, muscles, and circulation respond to services. The anatomy-versus-physiology distinction is a basic exam definition.
Source: Scientific Concepts — Physiology