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 \nSelect and explain. \nA. Failure rate / 1 women years, B. Failure rate/10 women years, C. Failure rate/100 women years, D. Failure rate/1000 women years', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Failure rate/100 women years', 'role': 'assistant'}]