POSSIBLE POINTS: 2 A student wondered if different types of plants would produce different amounts of oxygen. A pea plant, a fern, and a cactus were growing in equal-sized containers with equal amounts of soil. The student measured the amount of oxygen produced by each. Identify one factor (other than the size of the container and the amount of soil) that should be held constant in this experiment. Explain why this variable needs to be held constant.