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': 'What is atomic number : \nSelect and explain. \nA. Proton, B. Electrons + protons, C. Protons + neutrons, D. Protons + protons', 'role': 'user'}, {'content': 'Answer: A \nExplanation: A i.e. Proton Atomic number Number of protonsQ Number of electronQ Mass number No. of protons + No. of neutronsQ', 'role': 'assistant'}]