Answer the following question given this paragraph: 

Flatworms have a concentration of nerve tissue in the head end. This was a major step in the evolution of a brain. It was also needed for bilateral symmetry.


Q: Flatworms have a concentration of nerve tissue in the head end, which was a major step in the evolution of what organ?

 Choices:

- liver

- heart

- kidney

- brain

A:
brain
