photosynthesis means green plants convert from carbon dioxide and water and solar energy into carbohydrates and food and oxygen for themselves & if a process converts something to something else, then that something has the role of raw material in that process ->  in the photosynthesis process carbon dioxide has the role of raw material  photosynthesis is a kind of chemical process & if something is a raw material in a process then that something is required for that process ->  if something is a raw material in photosynthesis then that something is required for photosynthesis  in the photosynthesis process carbon dioxide has the role of raw material & if something is a raw material in photosynthesis then that something is required for photosynthesis ->  carbon dioxide is required for photosynthesis  if something is required for a process then that something is used in that process & photosynthesis is a kind of chemical process ->  if something is required for photosynthesis then that something is used in photosynthesis  carbon dioxide is required for photosynthesis & if something is required for photosynthesis then that something is used in photosynthesis -> carbon dioxide is used in photosynthesis