The correct answer is - B. Nationalism created ethnic tensions, resulting in civil war.
The nationalism in the early 20th century turned into a very dangerous weapon used by the politicians to control the people in accordance to their interests.
Through the usage of nationalism, the leaders of the countries managed to make the people feel as something special, as superior, as the best nation in the world, and in the same time promote hatred, despise, and putting blame on other ethnic groups for their troubles. Unfortunately, that mindset was what led to a terrible bloody war.
