A forest fire destroys trees in a large area. The forest rangers use the burned area to study the effect of different types of grasses on soil erosion. They choose ten plots of ground that are the same size. Each of these plots receives the same amount of sun. Each of these plots also has the same kind of soil and the same slope. The rangers plant each with a different type of grass. Measurements of soil erosion are made every week for the entire summer. Which is NOT controlled in the experiment A. the type of soil in the plots B. the size of the plots C. the amount of soil erosion D. the amount of sun the plots receive