The answer is the second choice: variation, overpopulation, adaptation
In the experiment, the independent variable is the drug amount. Experiment group is given certain amount of drug, control group is not given the drug. The dependent variable is the body fat. I think you means the control is the living environment including food and water.
An example of an ecosystem that benefits people could be the ocean.
The ocean regulates the temperature of the earth, allowing a more comfortable place for people to live.
Hope this helps!
Progesterone is a steroid hormone that is usually made by the adrenal cortex along with the gonads, which consists of the ovaries and testes
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