TOEFL Reading: ETS-TOEFL阅读机经 - INRO055343Y7DU3BL$

According to paragraph 4, why do birds prefer to sing at the beginning and end of the day? A. Their calls are less likely to produce echoes. B. Their calls do not have to compete with the sounds of other animals. C. They are less vulnerable to predators then. D. The sound of their voices travels farther.