Passage: Joseph notices that a certain water weed in his aquarium, called Elodea, made bubbles of oxygen when he turned on the light in the aquarium cover. He set up an experiment to see if changing the color of the light bulb in his aquarium affected the number of
bubbles produced by the weed.
Question: Which variable would Joe want to keep controlled (keep the same) in order to insure a fair test in his experiment?
the type of water weed
the wattage of the light bulb
the time he had the light bulb turned on
all of the above