sentences: photosynthesis is a source of oxygen
docs: photosynthesis is process. photosynthesis_in_plants is oxygenic_photosynthesis. oxygen is oxygen. photosynthesis is photo_chemical_process. gaseous_oxygen is oxygen. photosynthesis is chemical_process. oxygenic_photosynthesis is general_photosynthesis. general_photosynthesis is production. photosynthesis is process_by_which_leaf_makes_food.