Two test tubes were filled with a solution of bromothymol blue. A student exhaled through a straw into each tube, and the bromothymol blue turned yellow. An aquatic green plant was placed into each tube, and the tubes were corked. One tube was placed in the dark, and one was placed in direct sunlight. The yellow solution in sunlight turned blue, while the one in the dark remained yellow.

Which statement best explains why the solution in the tube placed in sunlight returned to the blue color?