Answer:
the speaker's not listening to the bird, because the bird isn't singing to him. At the same time, the speaker's also singing, which would make it difficult for him to listen to the bird even if the bird actually were singing to him.
Explanation:
Alphabetically, from A-Z and sometimes AA-ZZ
C. A teacher claims that student performance has gone down based on school - wide test results
Based on my interpretation of this website, I believe it's the astronomers. :)