Why not "nella biblioteca"? Is this like "in cucina" where the article can be omitted?
You can also write 'nella biblioteca' but you're right :) The article isn't necessary here. A few other examples: sono in cucina, siamo in chiesa, andiamo in campagna...
But only sometimes?
If so, is there a rule which applies?
What is more common language? Nella or in?
Why does the hint say "nella" but the right answer,"in"?