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': 'In calendar method of contraception, first day of fertile period is? \nSelect and explain. \nA. 10th day of shortest menstural cycle, B. 18th day of shortest menstural cycle, C. 10th day of longest menstural cycle, D. 18th day of longest menstural cycle', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Shortest period gives information about the first day of the fertile period. The shortest cycle minus 18 days gives first day of fertile period. For example, in a cycle of 28 days (most common), first day of fertile period will be 10th day of cycle (28 —18 = 10).\nLongest period gives information about the last day of the fertile period. The longest cycle minus 10 days gives the last day of fertile period. For example, in a cycle of 28 days, last day of fertile period will be 18th day of cycle (28 —10 = 18).', 'role': 'assistant'}]