TOEFL Reading: ETS-TOEFL阅读机经 - TS53TU_C5NRO_5NWL$

According to paragraph 6, what observation was made that weakened the reciprocal-altruism hypothesis? A. Some group members never spent any time guarding. B. There were some moments when no sentinel was present. C. No established order of guard duty existed. D. Some group members were less severely punished than others.