Why Have Ethnic Rivalries and Communal Violence Been Endemic in Decolonized

Why have ethnic rivalries and communal violence been endemic in decolonized African and Asian states?

A) The level of civilization in Africa and Asia was more primitive at the time of colonization.
B) Tribal life in Africa and Asia was traditionally more violent than other cultures.
C) The introduction of slavery by whites in the 19th century brutalized African and Asian culture.
D) Europeans hastily colonized Africa and Asia and established boundaries without reference to ethnic groups or cultural homogeneity.