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 most difficult source of infection among the following to control is \nSelect and explain. \nA. airborne, B. human to human, C. vector borne, D. vertical', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]