Why are control groups needed when testing living things

Me asap i need during photosynthesis, the following reaction takes place: carbon dioxide + water + light energy → sugar + oxygen during cellular respiration, the following reaction takes place: sugar + oxygen → carbon dioxide + water + chemical energy given this information, how are photosynthesis and cellular respiration related? question 2 options: a the products of photosynthesis are the reactants of cellular respiration. b photosynthesis and cellular respiration both start with the same materials. c photosynthesis and cellular respiration both produce the same materials. d the same type of energy is found in both photosynthesis and cellular respiration.