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': 'A family consists of 2 parents & 6 children susceptible to measles. There occurs a primary case of measles and 3 secondary cases within a short period of time. Secondary attack rate is \nSelect and explain. \nA. 60%, B. 38%, C. 67%, D. 50%', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]