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
Nucleus of an atom contains: 
Select and explain. 
A. Electrons, B. Only protons, C. Electrons and protons, D. Protons and neutrons
D: Protons and neutrons
Answer: D: Protons and neutrons 
Explanation: Ans. Protons and neutrons