Answer: 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.
