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': 'Failure rate of contraceptive method is determined by ? \nSelect and explain. \nA. Pearl index, B. Half life, C. Number of accidental pregnancies, D. Period of contraceptive practice continued', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e., Pearl index", 'role': 'assistant'}]