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: Social & Preventive Medicine
[{'content': 'Most common heavy metal poisoning in the world \nSelect and explain. \nA. Lead, B. Arsenic, C. Mercury, D. Cadmium', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]