TOEFL Listening: ETS-TOEFL听力机经 - RU87K4M9Q315VL35P$

According to the professor, why did Elizabeth Derryberry study the white-crowned sparrow? A. Its song has several regional dialects. B. Its behaviors are typical of those of other songbirds. C. Its evolution into two subspecies is well documented. D. Its song has been recorded over many years.