Hannah conducts an investigation to determine if adding fertilizer to the soil helps tomato plants grow taller. She puts 20 tomato plants in individual pots and adds an equal amount of soil to each pot. All 20 plants are the same height at the beginning of the investigation. She adds the same amount of fertilizer to the soil of 10 pots. Hannah measures the height of each plant once a day for 15 days. The presence of fertilizer in the soil is the
.

The height of the tomato plants is the
.

The pots without fertilizer are the
.