Plants tend to grow toward light. This is why plants placed on a sunny windowsill grow toward the window rather than experiencing a more even growth pattern. Mrs. Fairchild wants her houseplants to grow more evenly. She wonders if turning the houseplants will encourage even growth, but she doesn't know how often she needs to turn the plant, so she sets up the following experiment. Mrs. Fairchild purchases four identical plants. Each plant is about the same height and is planted in the same type of pot with the same type and amount of soil. She places a stick into each pot near the stems of the plants. The vertical sticks form a right angle with the top of the soil. Next, she places all four plants on the same windowsill, so they all receive the same amount of light each day. Plant 1 stays in the same position throughout the experiment. Plant 2 is turned 180 degrees every 12 hours. Plant 3 is turned 180 degrees every 24 hours. Plant 4 is turned 180 degrees every 48 hours. Mrs. Fairchild monitors the plants daily. She counts the number of leaves on each side of the plant, and she uses a protractor to measure how far each plant is leaning away from the stick. What is the test variable (independent variable) in the experiment?

A. the number of leaves on each side of the plant

B. the type of plant that was used

C. the distance that the plants leaned from the vertical stick

D. the amount of time that passed between turnings