To compare how different types of soil absorb water, Mei set up a science experiment. She poked holes into the bottoms of each of three plastic cups. She then filled one cup with sand, one with soil, and left the third cup empty. Next, she placed each cup in a stand over a beaker. Mei poured 400 milliliters (mL) of water into the cup containing the soil. She measured the amount of water collected in the beaker beneath the cup at one-minute (min) intervals for three min. Finally, she repeated the process two more times, first with the cup containing sand and then with the empty cup. Which of these conditions is NOT held constant in this experiment? A. the number of holes in each cup B. the time intervals C. the type of soil D. the amount of water