Answer:
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.
A number might be rounded when determining the number of people who will attend a basketball game. A number should not be rounded when determining the number of students who will participate in a chemistry lab. Only an estimate is needed for attendance at a basketball game because everyone will still get to see the game. In a chemistry lab the correct number of students should be known so that everybody has the proper equipment to use.
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Heat does not rise or fall because it is an energy being transferred. It is the hot air which rises because colder air is denser than hotter air.