TOEFL Listening: TPO-TOEFL阅读TPO - N3XGM98MK32NU03NN$

According to paragraph 2, which of the following presents a particular challenge to researchers who study play behavior in animals A. The delay between activities and the benefits the animal derives from them. B. The difficulty in determining which animal species play and which do not. C. The fact that for most animals, there is no clear transition from youth to full adulthood. D. The lack of research on the play behavior of animals other than canids and primates.