Which action would be most important to perform in a controlled experiment? ___A. Design an experiment that is based on previously published results. ___B. Expose the treated iron to the corrosive effects of the air. ___C. Test a large piece of treated iron and a small piece of untreated iron. ___D. Test a piece of treated iron and a piece of untreated iron under the same conditions.

D. Test a piece of treated iron and a piece of untreated iron under the same conditions.

Explanation:

A controlled experiment is defined as an experiment where the experimenter or the researcher tries to test or experiment his result by changing one variable at a time and keeping the other factors under the same conditions and sees how it effects the result.

The factors which can be changed in an experiment is called its variables.

In the context, testing a piece of treated iron as well as a untreated piece of iron under the same working conditions forms a controlled experiment.

wavelength variability is the right one