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: Pathology
Green discoloration on the surface of teeth is due to 
Select and explain. 
A. Porphyra, B. Internal resorption, C. Nasmynth membrane, D. Silver
C: Nasmynth membrane
Answer: C: Nasmynth membrane 
Explanation: None