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
Proton consists of: 
Select and explain. 
A. 2 up quarks + 1 down quarks, B. 2 up quarks + 2 down quarks, C. 1 up quarks + 2 down quarks, D. 1 up quarks + 1 down quarks
A: 2 up quarks + 1 down quarks
Answer: A: 2 up quarks + 1 down quarks 
Explanation: Protons are spin-½ fermions and are composed of three valence quarks, making them baryons (a sub-type of hadrons). The two up quarks and one down quark of a proton are held together by the strong force, mediated by gluons.