A scientist has 12 plants. All the plants were the same kind and about the same size. He put three plants on a window sill inside a room, three plants in a closet without a light and three plants outside on the ground. He put the last three plants outside too, but covered them with paper bags that had wholes punched in them for air. All the plants were given good soil and enough water. The plants on the window sill and the plants outside in the open grew well. The plants outside in the bags turned yellow and grew badly. The plants in the closet died. Which of the following is the best hypothesis based on the facts?