Answer:
Stem tubers grow from rhizomes or runners that swell from storing nutrients while root tubers propagate from roots that are modified to store nutrients and get too large and produce a new plant. Examples of stem tubers are potatoes and yams and examples of root tubers are sweet potatoes and dahlias.
Is this the answer your looking for?
Xylem transports water and minerals from the roots to the leaves of the plant, where it is used in the production of food.
Covalent bonds allow carbon atoms to form numerous compounds.
i think the answer is B :)
Answer - Theory
Reasoning - Your best bet and term to describe the evolution by natural selection is theory as to still scientific theory/research is still ongoing and changes can be made in a different direction.