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
Sensory receptors for pain are 
Select and explain. 
A. Pacinian corpuscles, B. End organs of Ruffini, C. End bulbs of Krause, D. Free nerve endings
D: Free nerve endings
Answer: D: Free nerve endings 
Explanation: None