Which sentence includes a nonrestrictive clause?

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