How can fossils be used to determine a relationship between living species and extinct ones?

Fossils are significant in showing the evolution of species. Fossils can be used to determine features that are shared between the current living organisms and ancient organism which they are related. This can be used to study how organisms evolve and how this is related to the ecological conditions of the time compared to now. Organisms evolve so they can adapt to a changing habitat.  

answer; /// i believe the correct answer is (reactants);

a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring.