The text suggests that investigators of monkeys’ social behavior have been especially interested in aggressive behavior among monkeys because ______.

D
细节题。文章第一段最后一句提到“Studies directed at describing aggressive behavior and the situations that elicit it…were therefore among the first investigations of monkeys’ social behavior”，由therefore可知前面既是研究人员选择研究该项目的原因，文章首句提到“Investigators of monkey’s social behavior have always been struck by monkeys’ aggressive potential and the consequent need for social control of their aggressive behavior”，对应选项D，故D项为正确答案。