TOEFL Reading: ETS-TOEFL阅读机经 - _4G4YSG37Y567XN5L$

According to paragraph 1, which of the following is true of sociality among animal species? A. Sociality is much more common among invertebrates than among vertebrates. B. Very few animals are considered social because most spend the majority of their lives alone. C. An animal group must contain more than one family unit for the species to be considered social. D. All animals that rear their young are considered to be social.