One student did an experiment to find out if fertilizer can make plants grow taller. The steps of the experiment are listed below. Take three pots of the same size. Fill the pots completely with the same type of soil. Plant tomato seeds in the pots of soil. Add different amounts of fertilizer to the three pots. Keep the first pot inside the house, the second pot under the shade of a tree, and the third pot in direct sunlight. Add the same amount of water to the pots every day. Measure the heights of the plants after regular intervals. The results of the experiment were not valid. Which of these statements best describes a flaw in the experiment?