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: Medicine
[{'content': 'Period of isolation in salmonella infection is till \nSelect and explain. \nA. 72 hours after chemotherapy, B. Widal negative, C. 3 consecutive stool cultures are negative, D. Urine culture is negative twice', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]