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
Substance P is increased is response to pain in periphery, by which of the following? 
Select and explain. 
A. Mast cells, B. Endothelium, C. Plasma, D. Nerve terminals
D: Nerve terminals
Answer: D: Nerve terminals 
Explanation: Substance P seen at

Primary pain nerve endings in dorsal horn of spinal cord
Hypothalamus
Nigrostrial pathway of basal ganglia