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': 'Safe period is calculated by - \nSelect and explain. \nA. Length of menstrual cycle, B. DurationDate of ovulation, C. Length of ovulation, D. Length of luteal phase', 'role': 'user'}, {'content': "Answer: A \nExplanation: Safe Period (rhythm method) \n\nThis is also known as the calendar method first described by Ogino.\nThe method is based upon the fact that ovulation occurs from 12 to 16 days before the onset of menstruation.\nCalculation is as follows :\n\n\nThe shortest cycle minus 18 days gives the first day of the fertile period.\nThe longest cycle minus 10 days gives the last day of fertile period.\nFor example, if a woman's menstrual cycle varies from 26-31 days, the fertile period during which she should not have intercourse would be from the 8th day to 21st day of the menstrual cycle, counting day one as the first day of the menstrual period.", 'role': 'assistant'}]