sentences: if an organism uses a process to convert something to something else, then that organism is a source of that something else through that process & photosynthesis means green plants convert from carbon dioxide and water and solar energy into carbohydrates and food and oxygen for themselves ->  plants are a source of oxygen through photosynthesis  plants are a source of oxygen through photosynthesis & a source of something increases the amount of that something -> plants increase the amount of oxygen