Introduction.
A book's introduction is closely related to the content of the book itself. Usually found in nonfiction work, the introduction may summarize the main argument presented in the rest of the book, define any important terms, or fill in background details.
An abstract noun is a noun that you can't feel, hear, taste, smell, and see. Like hope. A concrete noun is a noun that you CAN feel, hear, taste, smell, and see.
So the abstract noun in this sentence is not choir, but harmony.
hope it helps!
What is in bold than the answer will come
