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 ingroup/outgroup conflict because there were no intrinsic differences between the Rattlers and the Eagles.
B) situations that required cooperation and the joint efforts of both groups reduced the conflict and hostility between them.
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.