Local officials are concerned that agricultural fields at site B are increasing sedimentation in the river because of soil eroding from the fields. They hire an environmental consulting company to perform a study to determine if this is true. The company marks out two one-hectare plots at point B and point C in the diagram. The characteristics of the land in both plots are identical except that plot B is covered by agricultural fields and plot C is covered by undisturbed forest. Rain gauges are set up at the up-slope edge of the plots, and devices that collect eroded soil are set up along the down-slope edge of the plots. The environmental consultants hypothesize that more soil will be eroded from plot B than from plot C. Data are collected at the rain gauges and soil-collection devices at the end of each month for a year. (i) Describe how plot C serves as the control in the study.