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': 'Nucleus of an atom contains: \nSelect and explain. \nA. Electrons, B. Only protons, C. Electrons and protons, D. Protons and neutrons', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Ans. Protons and neutrons', 'role': 'assistant'}]