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
NMR based in the principle of ? 
Select and explain. 
A. Electron beam, B. Proton beam, C. Magnetic field, D. Neutron beam
B: Proton beam
Answer: B: Proton beam 
Explanation: B i.e. Proton Beam