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': 'Pearl Index is expressed \nSelect and explain. \nA. Per 10 woman years, B. Per 1000 woman years, C. Per 100 woman years, D. Per 50 woman years', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]