Emilio's teacher told his class that a controlled experiment's results are valid only if one factor in the experiment is changed and all the other factors remain constant. why is this statement true? a. when only one factor is changed, you can be more certain that it caused the results. b. when only one factor is changed, you don't need to use a control. c. changing several different factors in an experiment takes too many controls. d. changing several different factors in an experiment takes too much time.