This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Which one of the following molecules is used for signalling? 
Select and explain. 
A. CO2, B. O2, C. NO, D. N2
C: NO
Answer: C: NO 
Explanation: NO synthesized from arginine in endothelial cells and it acts stimulation of soluble guanylyl cyclase and generation of cGMP producing relaxation in vascular smooth muscle cells. It may be the signal by which postsynaptic neurons communicate with presynaptic endings to enhance release of glutamate. It may also play a role in synaptic plasticity and thus in memory and learning. NO is also necessary for the antimicrobial and cytotoxic activity of various inflammatory cells. In the gastrointestinal tract, NO is impoant in the relaxation of smooth muscle. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 7. Neurotransmitters & Neuromodulators. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.