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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.

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.