The chaffinch is a type of European songbird, and were gonna use the chaffinchs song development as illustrative of songbirds song development in general, because many other songbirds follow this same pattern, this pattern of learning songs.

看文中关键词，反复重复的都是song, development所以这就是本文要说的主旨。而我刚讲的定位找AA，答案找AB就是这样一个理论。托福做题的过程，就是一个寻找同义词改写的过程。OG里面的第一个conversation，里面的学生说I have some quick questions about how to write up the research project I did this semester, about...climate varation. 原文中的定位词相当于是write the project。再看正确答案中的write the report。write就是我讲的AA，而project就是我说的AB，同义词替换。

那么我们回头再看一下关于song development的lecture，song development就是AA。而答案中的How it develops from an early age就是AB，第二个位置，定在了关于subsong的举例的位置，因为举例属于supporting detail看到了么baby反复重复，所以选B

23. How does the professor introduce her description of the sounds a chaffinch makes during subsong?

She contrasts the subsong of a chaffinch with the subsongs of other songbird species.

She compares the sounds a chaffinch makes to those a human baby makes.

She describes the sound of an adult chaffinchs full song.

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