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
[{'content': 'Which is the most common pigmentation of oral cavity \nSelect and explain. \nA. Racial pigmentation, B. Amalgam tattoo, C. Fluorosis, D. None of the above', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]