Mrs. Hendren has decided that she would like to grow some tomatoes and wants to know how much light is the best amount, so she plants tomato seeds in 4 different pots and leaves one pot in the dark, she gives 6 hours of light a day to the second, eight hours of light a day to the third, and ten hours of light a day to the fourth. After 3 months she measures the size of the tomatoes grown. What is the independent variable?​