Scientists are studying fertilizers to help grass grow in southern California. Fertilizer is any substance that can be added to soil to help plants grow in it. In other words, fertilizer makes soil more fertile. The scientists set up different plots of land that all use the same type of soil, but they use different types of fertilizer in each plot. Scientists then measure the grass growth in the different soils over time.

How should scientists set up the control group in this experiment?