photosynthesis is a source of / makes food / energy for the plant by converting carbon dioxide, water, and sunlight into carbohydrates & a plant absorbs carbon dioxide from the air to perform photosynthesis ->  a plant absorbs carbon dioxide to make food for itself  if a living thing takes in something to perform a process then that something is required for that process & absorbing something is a kind of method for taking in something ->  if a living thing absorbs something to perform a process then that something is required for that process  a plant absorbs carbon dioxide to make food for itself & if a living thing absorbs something to perform a process then that something is required for that process ->  carbon dioxide is required for a plant to make food for itself  carbon dioxide is required for a plant to make food for itself & carbon dioxide is a kind of gas -> carbon dioxide is a kind of gas that is required by a plant to make food for itself