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Given a scientific passage and an answer, generate a question for the given answer.

Passage: Scientific experiments involve controls , or subjects that are not tested during the investigation. In this way, a scientist limits the factors, or variables that can cause the results of an investigation to differ. A variable is a factor that can change over the course of an experiment. Independent variables are factors whose values are controlled by the experimenter to determine their relationship with an observed phenomenon. Dependent variables are the observed phenomenon, and change in response to the independent variable. Controlled variables are also important to identify in experiments. They are the variables that are kept constant to prevent them from influencing the effect of the independent variable on the dependent variable. Answer: constant
Output:
Controlled variables are kept what to prevent them from influencing the effects of the independent variable on the dependent variable?