Sally wants to test how the type of light affects the height of bean plants. She uses three different types of light bulbs: LED, incandescent, and CFL. She exposes the plants to the lights for the same amount of time and plants the seeds in containers that are the same size, with the same type of soil, and waters them with the same amount of water each day. She measures the height of
the plants using a ruler and records the growth each day. Which of the following is the independent variable in the experiment?