Which sentence includes a nonrestrictive clause?

Camels, which sway from side to side as they walk, are often called ships of the desert.
For centuries, camels have been used to transport people and cargo through the desert.
Not only do they need very little water, but camels can drink water that is salty or bitter.
Camels can live in extreme conditions, such as in areas with extreme heat, aridity, and a lack of vegetation.

camels,which sway from side to side as they walk,are often called ships of the desert

Explanation:

This answer includes a piece of information that can be removed without altering the meaning of the sentence. It could have just said camels are often called ships of the desert. The added information about the camels swaying as they walked is additional information but it isn't important to what's being said, which makes this answer a nonrestrictive clause.

op: object of preposition

  this riddles' answer is library.

  the "leaves" are a reference to the books' pages that the library contains; the branches are a network of libraries that connect multiple libraries' collections, resources, and items in a system of a common database, integrating them; the branches' organization are a useful management tool for coordinating the multiple collections many libraries have.