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': 'On X-ray, right border of the mediastinum is formed by all the following except: \nSelect and explain. \nA. Superior vena cava, B. Right atrium, C. Right ventricle, D. Right brachiocephalic vein', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. Right ventricle', 'role': 'assistant'}]