A student performed an experiment to determine the effect of acid rain on plant growth. The student divided 80 seeds of a fast-growing plant species into four groups of 20 seeds each. The four groups of seeds were then planted in soil. After all of the seeds germinated in the same growing conditions, they were watered once a day with simulated rainwater that had different levels of acidity. The acidity of the four different watering solutions was as follows: neutral water, 0.1%, 1.0%, and 10%. The average height of the plants in each group was measured over a two-week period. Required:
Identify the independent variable in the experiment.