If three variables were changed at one time, what would happen to the accuracy of the conclusions made for an experiment?
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Do an experiment, get a result. Investigate the process. Change one variable, get a different result. Reasonably, the change in the one variable is responsible for the different result. Change three variable, and get a different result, you know that the change in ONE of the variables led to a different result, but WHICH variable of the three you changed? So you have to do three separate experiments to see what variable precipitated the change.