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
Which is the most common pigmentation of oral cavity 
Select and explain. 
A. Racial pigmentation, B. Amalgam tattoo, C. Fluorosis, D. None of the above
A: Racial pigmentation
Answer: A: Racial pigmentation 
Explanation: None