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': 'Velocity of x-rays are: \nSelect and explain. \nA. 1 lakh miles/sec., B. 2 lakh miles/sec., C. 3 lakh miles/sec., D. 0.5 lakh miles/sec.', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]