TOEFL Listening: TPO-TOEFL阅读TPO - 2P1JX3A0525216E3N$

According to paragraph 2, which of the following is true about the circadian periods of animals deprived of environmental cues? A. They have the same length as the daily activity cycles of animals that are not deprived of such cues. B. They can vary significantly from day to day. C. They are not the same for all members of a single species. D. They become longer over time.