To observe how plants compete for space, seeds were tested in an experiment. The same amount of soil was put into each of five plastic cups. Two seeds were planted in the first cup, 4 seeds were planted in the second cup, 8 seeds in the third cup, and 16 seeds were planted in the fourth cup. Only one seed was planted in the last cup to compare with the other pots. After 25 days, the height of the plants was measured to determine which plants grew best. what is the control of this experiment