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D. All variables are measured to provide data for the experimental results

Explanation:

In an experiment, variables are changed from one part of the experiment to another to see the change in effect so option A is incorrect.

Not all variables are changed in experiments. Only independent variables are changed so that their differential effects on the dependent variable is measured. Option B is therefore incorrect.

Not all variables affect the results o the experiment so option C is incorrect as well.

Variables are indeed measured so that data is produced for the results which would enable conclusions to be made so option D is correct.

clumped distribution, because you'de have all of the population gathered in a relatively small are.

it's the most common because it provides protection and a mate for reproduction

example: humans, seeweed, cattles, etc.