A student performed an experiment to determine the relationship between air temperature and growth in plants. She divided 24 of the same type of seedlings into four groups of six plants. The plants were potted in identical pots with the same type and amount of soil. She grew each group at a different temperature; Group A was grown at 4°C, Group B was grown at 13°C, Group C was grown at 22°C, and Group D was grown at 31°C. Each pot received 40 mL of water every other day. She recorded the height of the plants in each group after a four-week period and found the mean height of each group.
What is the independent variable in this experiment?
height of the plants
temperature the plants are grown in
type of plant
amount of soil