I think the answer is c but I looked it up and got this: system or thing used as an example to follow or imitate.
Some don't have any able to see effects,but some cause diseases

In humans, the medulla region of the brain is mainly responsible for the regulation process of respiration. It is also called the respiratory rhythm centre. The chemosensitive area located near the respiratory centre sends signals across nerve impulses to alter the rate of expiration to eliminate compounds.
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When populations have differences in rituals or behaviors. When populations are separated by rivers, mountains, r bodies of water.
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Answer:
Stimulation of thoracic or lumbar spinal regions; impulse reaches chain ganglion; acetylcholine release
Explanation: