Historians seek to interpret historic sites, artifacts, written records, and verbal histories without adding their own what?

ideas of the historic background
theory they hope to prove
biases of interpretation
narrative of the course of events

The correct answer is - Biases of interpretation.  

In general, the historians are trying to avoid adding their biases of interpretation of the history. They try to focus on using as information only the sites, the artifacts, what has been written, the verbal history among the peoples, in order to give the most objective interpretation of the historical events.  

Unfortunately, there's also lot of historians that do tend to use their biases of interpretation of the history, often resulting in nationalistic ideals, tensions between nations and people, and even war if they are convincing enough.

the population grew a lot and was making more money