Elodia is a plant that lives in water. Like all plants, elodea can make its own food via photosynthesis. Oxygen is a gas produced during photosynthesis. Photosynthesis can only happen in the presence of light. To measure the rate of photosynthesis, a piece of elodia can be placed in an inverted test tube filled with sodium bicarbonate solution. Oxygen gas produced by photosynthesis will collect at the top of the tube. The experimental set up created by a student is shown above. What is the dependent variable in this experiment?