And an experiment which variable changes in response to the manipulation of another variable

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."

photosynthesis makes the glucose that is used in cellular respiration to make atp. the glucose is then turned back into carbon dioxide, which is used in photosynthesis. while water is broken down to form oxygen during photosynthesis, in cellular respiration oxygen is combined with hydrogen to form water.