Which of the Following Sentences Contains a Nonrestrictive Clause

Which of the following sentences contains a nonrestrictive clause?

A) The Soviet Union collapsed by the end of the twentieth century.
B) Treadmills that are well built can be a great way to exercise.
C) The boy who cried wolf received no help in the end.
D) Kira hit the boy, who was insulting her.
E) In my opinion, Christopher Walken is a surprisingly good dancer.