This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Demography and family planning, Subject name: Social & Preventive Medicine
[{'content': 'Pearls index \nSelect and explain. \nA. Per 10 woman years, B. Per 100 woman years, C. Per 1000 woman years, D. Per 10,000 woman years', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Pearl index is defined as the number of failures per 100 woman years of exposure (HWY) Failure rate per Hundred Women Years = Total accidental pregnancies X 1200/ Total months of exposure Reference : Park&;s textbook of preventive and social medicine, 23rd edition, Page no: 510', 'role': 'assistant'}]