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: Radiology
[{'content': 'Reduced salivary flow following irradiation is dose dependent. At what dose does the flow reach essentially zero: \nSelect and explain. \nA. 4000 rads, B. 5000 rads, C. 6000 rads, D. 7000 rads', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]