Two test tubes are filled with a solution of bromthymol blue. a student exhales through a straw into each tube, and the bromthymol blue turns yellow. an aquatic green plant is placed in each tube, and the tubes are corked. one tube is placed in the dark, and the other tube is placed in direct sunlight. the yellow solution in the tube in sunlight turns blue, while the one in the dark remains yellow. which statement best explains why the solution in the tube placed in sunlight returns to a blue color? a) carbon dioxide was produced by respiration. b) carbon dioxide was removed by photosynthesis. c) oxygen was produced by photosynthesis. d) oxygen was removed by respiration.