Brainwashing. When people are strayed away from society and kept in a specific groups. They are conditioned to believe what they are hearing, what they see, and what their told to believe.
Once they start truly “believing the truth” of what their cult leader is practicing or what their preaching it is hard to believe anybody else.
There are multiple choices for this Answer.
A. Encourage trade in Europe
.
D. Create a single European culture.
A and D are correct.
The correct answer is deadly governmental "purging".
Examples for this are: Holocaust - the systematic extermination of Jews and Roma in Germany and the territories occupied by them, and the killings of Jews in other places, such as the Soviet Union.(less known, but also happened)