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: Surgery
[{'content': 'True about goodsall rule? \nSelect and explain. \nA. To know about likely position of external opening of fistula in ano, B. Posterior located external openings are joined by straight line, C. Not valid if external opening >3cm away from anal verge, D. Not valiod in Single external opening', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Goodsall rule is to know internal opening of fistula in ano.', 'role': 'assistant'}]