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
What is atomic number : 
Select and explain. 
A. Proton, B. Electrons + protons, C. Protons + neutrons, D. Protons + protons
A: Proton
Answer: A: Proton 
Explanation: A i.e. Proton Atomic number Number of protonsQ Number of electronQ Mass number No. of protons + No. of neutronsQ