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 defined as \nSelect and explain. \nA. Accidental pregnancies per 1000 women-years of exposure, B. Accidental pregnancies per 100 women-years of exposure, C. Accidental pregnancies per 10 women-years of exposure, D. Accidental pregnancies per women-years of exposure', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]