A student places six plants of equal sizes into six different, equal-sized pots containing the same amounts of potting soil and labels the pots U, V, W, X, Y, and Z. Using two pots in each of the three experiments, the student varies the amounts of sunlight, water, or air one plant receives but leaves the other pot unchanged. The tables show the factor varies in each pot and the observations made by the student after one week. Which conclusion best explains the connection among all three experiments?