I believe it is simple and merry christmas to you too
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!
Catching fish is one of the oldest pastimes.
fish is a noun
B) to show omitted words in quotes passages