第15题: [单项选择]According to the passage, early humans ______. A. were less selfish than the present-day people B. were more co-operative in nature C. had to be co-operative to survive D. had to behave better than the present-day people 参考答案:C 答案解析:本题是细节题,我们可从原文中找到对应:For social groups to work,individuals had to co-operate,rather than act selfishly