Answer: D
Explanation:
In a controlled scientific experiment, there should be an independent variable, a dependent variable, and a control. The independent variable is the factor in the experiment that is manipulated by the scientist. The dependent variable is the factor in the experiment that changes in response to the independent variable. Controls in an experiment are used as comparison factors, and they can help determine the magnitude of the experiment's results.
Scientists carefully control their variables so they can understand how the variables affect the system as a whole. This makes the data collected from an experiment meaningful.