B.To Understand group behavior through observation
True, DNA is DNA, it matters not wether or not it belongs to a plant, you could mix a carrot with a pig (not sure why you would, but you can)
Answer:
At the bottom you have the producers aka auto trophic livings (which use inorganic compounds to grow, produce energy, matter and so on..). These organisms would be plants, Cyrano bacteria, over all any organisms that do photosynthesis.
Studying cells helps us to construct better medicines and vaccines, which is beneficial to understanding life.
