Final answer:
c. carbon monoxide
Step-by-step explanation:
The correct answer is c. carbon monoxide. While nitric oxide and acetylcholine are both used by the body as neurotransmitters, carbon monoxide is not used in this capacity. Nitric oxide acts as a paracrine chemical that causes vasodilation, while acetylcholine is used as a neurotransmitter in the neuromuscular junction and the autonomic nervous system. Carbon monoxide, on the other hand, is a toxic gas that binds to hemoglobin in red blood cells, preventing them from carrying oxygen.