This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
[{'content': 'Among the above characteristics, which are used to measure the ability of biological agents to induce clinically apparent illness? \nSelect and explain. \nA. 1 and 2, B. 2 only, C. 3 only, D. 2 and 3', 'role': 'user'}, {'content': 'Answer: A \nExplanation: .', 'role': 'assistant'}]