Fred is conducting a controlled experiment to test whether using a grow light at night will help a tomato plant grow faster. Below are the steps to Fred's experiment: Plant tomato plants in two separate, but identical, flowerpots. Place the flowerpots in the same area outside during the day. At night, bring both flower pots inside and place one in the dark and one under a grow light. Give the plants the same amount of food and water each week. Measure the growth of the plants. Draw conclusions based on the data collected. Which of the following variables must Fred hold constant in this experiment?