Students want to determine how vegetation affects soil erosion. They collect four soil samples of equal starting mass with different types of vegetative cover, as seen in the table below. Each sample is placed in a three-sided plastic dish that can drain into a beaker and is exposed to the same amount of consistent, steady rainfall for ten minutes. The water and soil runoff are collected in the beaker and separated. The average values for each sample are provided below. Which of the following is the control group for this experiment?