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: Dental
[{'content': 'The “Shoe Leather Survey” was given by \nSelect and explain. \nA. Trendley H. Dean, B. Frederick McKay, C. Greene Black, D. J. M. Eager', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Fluoride was established as the causative factor for mottling of enamel through the historical studies conducted by Trendley H. Dean, known\xa0 as the "Shoe Leather Survey".', 'role': 'assistant'}]