if something releases something else then that something is the source of that something else & plants are a source of oxygen through photosynthesis ->  plants release oxygen during photosynthesis  a plant absorbs carbon dioxide from the air to perform photosynthesis & to absorb something can mean to take something in ->  a plant takes in carbon dioxide to perform photosynthesis  plants release oxygen during photosynthesis & a plant takes in carbon dioxide to perform photosynthesis -> plants take in carbon dioxide and release oxygen during photosynthesis