Answer the following question given this paragraph: 

Living chordates are mainly vertebrates. In vertebrates, the notochord develops into a backbone, or vertebral column, after the embryonic stage. A small percentage of chordates are invertebrates. Their notochord never develops into a backbone. Invertebrate chordates include tunicates and lancelets. Both groups of animals are small and relatively primitive. They are probably similar to the earliest chordates that evolved more than 500 million years ago.


Q: During the embryonic stage of vertebrates, the notochord develops into what?

 Choices:

- rib cage

- umbilical cord

- brain stem

- backbone

A:
backbone
