How has the diversity and ethnic nationalism in Europe caused tension, especially in the 20th century?

sonar maybe? ? i dont know the choices

d.

the northwest territory became the most densely populated area in the u.s.

i believe it would influence their own way of understanding things. it shall be a foundation where they can draw or start an argument.   historians are also  scientists that  have their tools of analysis and expertise to unlock a variety of difficulties such as relationships of man, origin, politics, economy, and culture. based on facts and understanding on these areas, they may shade light to extract ideas and develop options to validate a hypothesis or debunk it.