Muzafer Sherif's Work with 11-Year-Old Boys at a Summer Camp

Muzafer Sherif's work with 11-year-old boys at a summer camp demonstrated that:

A) it was very difficult to establish in-group/out-group conflict because there were no intrinsic differences between the Rattlers and the Eagles.
B) when situations required cooperation and the joint efforts of both groups, conflict and hostility between them were reduced.
C) simple social contact between the two groups greatly reduced conflict and hostility.
D) contrary to expectations, competition between the two groups actually increased harmony between them.