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': 'Cluster testing is useful in detecting cases of \nSelect and explain. \nA. Measles, B. Unimmunized children, C. Sexually transmitted infections, D. Completely immunized children in the age group 12-23 months', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]