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
Reduced salivary flow following irradiation is dose dependent. At what dose does the flow reach essentially zero: 
Select and explain. 
A. 4000 rads, B. 5000 rads, C. 6000 rads, D. 7000 rads
C: 6000 rads
Answer: C: 6000 rads 
Explanation: None