In an experiment, 90 newborn flies are divided evenly into three separate groups and placed in boxes inside an incubator that is maintained at a constant temperature of 30c. on a daily basis, the first group is given water containing 1mg/ml of sugar; the second is given water containing 2mg/ml of sugar; and the third is given water containing 3 mg/ml of sugar. after weeks, the average length of the flies in each group is determined. which factor is the independent variable in this experiment