Dr. Brosnan and Dr. de Waal have eventually found in their study that the monkeys ____

Dr. Brosnan and Dr. de Waal have eventually found in their study that the monkeys ________.

[A] prefer grapes to cucumbers

[B] can be taught to exchange things

[C] will not be cooperative if feeling cheated

[D] are unhappy when separated from others

请帮忙给出正确答案和分析，谢谢！