What is the control group and the controlled variables in this experiment?

Agricultural scientists were concerned about the effect of air pollution, sulfur dioxide in particular, on soybean production in fields adjacent to coal-powdered power plants. Based on initial investigations, they proposed that sulfur dioxide in high concentrations would reduce reproduction in soybeans. They designed a experiment to test this hypothesis. In this experiment 48 soybean plants, just beginning to produce flowers, were divided into two groups, treatment, and no treatment. The 24 treated were divided into four groups of 6. One group of 6 treated plants was placed in a fumigation chamber and exposed to 0.6 ppm of sulfur dioxide for 4 hours to stimulate sulfur dioxide emissions from a power plant. The experiment was repeated on the remaining three treated treated groups. The no-treatment plants were returned to the greenhouse. When the beans matured, the number of bean pods, the number of seeds per pod, and weight of pods were determined for each plant.|||The controlled variable is the exposure vs. non-exposure to sulfur dioxide. The control group i.e. the no-treatment plants, was not exposed to sulfur dioxide.