Use the scenario to answer the question.
A student decides to test the effects of watering on plants. She chooses 6 tomato plants and places all of them in the same part of a garden with plenty of space between them. They get the same amount of sunlight and the same amount of fertilizer each day. She gives each plant a different amount of water, with the first plant getting no water and the others getting 200 mL, 400 mL, 600 mL, 800 mL, and 1,000 mL. She records the total height of the plants each day for 3 weeks.
What is the independent variable in this experiment?
sun exposure
plant height
amount of fertilizer
amount of water