To compare how different types of soil absorbed water, Christian set up an experiment. He poked equal numbers and sizes of holes into the bottoms of three plastic cups. He filled one cup with sand, one with potting soil, and left one empty. He placed each cup in a stand over a beaker. Christian poured 400 mL (milliliters) of water into each of the cups. He then measured the amount of water collected in the beakers beneath each cup at 1 minute intervals for 3 minutes. What is the control in this experiment? Group of answer choices