Answer:
X-Also known as the Mitochondria
"C. a toy train" is the item that has characteristics similar to a scientific model Scientific models do a reasonably good job of predicting what will happen in the real world, much like a toy train following a track.
The answer is Overproduction
By definition in biological terms, overproduction means that each generation has more offspring that can be supported by its environment and it results to competition for limited resources.
The survival of the fittest's main idea is that the organisms or species that are well adapted to their environment have higher chances of survival.
The descent with modification proposes the idea that species undergo changes over time that produces new species in whcih they share a common ancestor.
Adaptation means organisms learn to adjust to their new environment.
Gene..
hope this helps. :)