This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Physics of Radiotherapy, Subject name: Radiology
[{'content': 'Air on X-rays appears - \nSelect and explain. \nA. White, B. Black, C. Grey, D. None of the above', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. is 'b' i.e., Black o Structures that are dense (such as bone) will block most of the x-ray particles, and will appear white.o Metal and contrast media (special dye used to highlight areas of the body) will also appear white.o Structures containing air will be black, and muscle, fat, and fluid will appear as shades of gray.", 'role': 'assistant'}]