He tells his students that in the help coordinate contraction of certain muscles and relaxation of others. Answers: a. motor neurons; peripheral nervous system b. motor neurons; spinal cord c. sensory neurons; peripheral nervous system d. interneurons; spinal cord

A. motor neurons; peripheral nervous system

Explanation:

Motor neurons carry signals from the peripheral nervous system to the muscles. This causes the muscles to either relax or contract. Note that the peripheral nervous system consists of neurons and parts of neurons outside of the central nervous system which consists of the brain and spinal cord. When signals are transmitted between the motor neurons and peripheral nervous system, this causes a response to stimulus in the muscles which would either, relax or contract.

the answer would be 3