Answer:molecule
Explanation:A molecule is the smallest particle of a compound that still has the compound's properties. It consists of two or more atoms bonded together
The central nervous system is made up of the brain and spinal cord. The peripheral nervous system is made up of nerves that branch off from the spinal cord and extend to all parts of the body.
I believe B is the answer..
The site for gas exchange in the lungs is the alveoli.