What Has Research Shown About In-Groups and Out-Groups

What has research shown about in-groups and out-groups?

A) In-group-out-group distinctions actually promote racial harmony.
B) In-group-out-group distinctions inhibit ethnocentrism.
C) In-group-out-group distinctions give people an unclear sense of their place in a world of diversity.
D) In-group-out-group boundaries lead to unfairly negative views of out-groups.
E) In-group-out-group boundaries are inevitable.