Eric and Chris run a non-regulated natural monopoly producing electricity for a small town. The barrier most likely preventing other firms from competing with her is

increasing returns to scale

Explanation:

The biggest barrier for other firms are increasing returns to scale. This is because Eric and Chris have their company already established and also have their clientele all hooked up and using their service. This allows them to produce a much higher electrical output for their clients with a certain Income. Newer companies will need a much higher income just to be able to produce a similar electrical output in order to try and compete with Eric and Chris.

answer/// product differentiation;