In an experiment to determine the effect of light on plant growth, researchers divided 40 identical tomato plants into four groups of 10 plants each. Group A received 12 hours of light per day, Group B received 8 hours, Group C received 4 hours, and Group D was kept in complete darkness. All plants were watered equally and kept at the same temperature. After four weeks, the researchers measured the average height of the plants in each group.

Which of the following best describes a flaw in this experimental design?