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': 'Which is not mutagenic? \nSelect and explain. \nA. X-rays, B. UV rays, C. Ultrasound, D. Beta rays', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. Ultrasound', 'role': 'assistant'}]