2. The changing climate might affect the amount of rain that falls during different seasons. Eighteen plots of open grassland, each with an area 70 square meters, were available for a study. One response variable is total plant biomass produced in a plot over a year. Kenwyn Suttle of the University of California at Berkeley and his coworkers wanted to compare the effects of three treatments: added water equal to 20% of annual rainfall elther during January to March (winter) or during April to June (spring), and no added water (control). Explain why it was necessary to include a control group that didn't get additional water.