A scientist wants to perform a controlled experiment to test the effect of rust on chemically treated iron. Which action would be most important to perform in a controlled experiment?

A controlled experiment changes only the factor being tested—in this case, whether the iron has been chemically treated—while keeping other conditions the same. Testing treated and untreated pieces under the same conditions makes it possible to compare how much they rust. Choice D, testing both pieces under the same conditions, is therefore the most important action.

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