Walter wanted to see if the amount of sunlight that sunflowers get changes how tall they grow. So he took three different sunflower plants, all potted in the same soil, received the same amount of water, and exposed them to different amounts of sunlight. Plant A was exposed to 12 hours of sunlight, the typical amount of sunlight that sunflower plants typically receive. Plant B was exposed to 6 hours of sunlight and Plant C was exposed to 3 hours of sunlight. Walter observed the plant growth over the next three weeks and recorded the heights of each plant.
What is the controlled sample?