According to the author, studies such as Southwick's had which of the following effects on investigators theories about monkeys' social behavior? A. They suggested that existing theories about the role of aggression among monkeys did not fully account for the monkeys' ability to maintain an established social order. B. They confirmed investigators' theories about monkeys' aggressive response to competition for food and water. C. They confirmed investigators' beliefs about the motivation for continued aggression among monkeys in the same social group. D. They disproved investigators' theory that the introduction of intruders in an organized monkey group elicits intragroup aggressive behavior. E. They cast doubt on investigators' theories that could account for observed patterns of aggression among monkeys.