Answer the following question given this paragraph: 

Chordates include vertebrates and invertebrates that have a notochord. Chordates also have a post-anal tail, dorsal hollow nerve cord, and pharyngeal slits. Vertebrate chordates have a backbone, while invertebrate chordates do not. Invertebrate chordates include tunicates and lancelets; both are primitive marine organisms.


Q: What do vertebrate chordates possess that invertebrate chordates do not?

 Choices:

- parasite

- Thumb

- Milk production

- backbone

A:
backbone
