TOEFL Reading: ETS-TOEFL阅读机经 - 9EZ7IE76853DQHFIL$

Why does the author mention that "it is very unusual to hear a wild bird singing a song which is not typical of its own species"? A. To explain why a variety of different bird songs are often heard in a relatively small area B. To argue that social constraints have a greater impact upon learning than do genetic constraints C. To provide an example of how the process of learning rules varies from one species to another D. To illustrate how effective the different constraints upon learning are in young birds