In an experiment to determine the effects of radiation on the germination of seeds, a factor that has to be controlled is

a. the production of carbon dioxide through the process of photosynthesis

producers: apple trees, potatoes, carrots, grains, acorns, cabbage, grass, corn, lettuce

consumers: deer, mice, rabbits, chickens, squirrels, horses, worms, cows

predators: eagles, frogs, wolves, owls