This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Radiology
[{'content': 'NMR is based on the principle of \nSelect and explain. \nA. Electron beam, B. Proton beam, C. Neutron beam, D. Magnetic field', 'role': 'user'}, {'content': "Answer: D \nExplanation: The phenomenon of 'nuclear induction', later to be termed 'nuclear magnetic resonance' (NMR), was described by Bloch and Purcell and their colleagues in 1946. NMR has been used extensively for studying the propeies of matter at the molecular level (NMR spectroscopy). In applications to medicine, the term 'nuclear' has been dropped, leaving it as magnetic resonance or MR(Ref: Grainger&;s diagnostic radiology, 6th ed; page 90)", 'role': 'assistant'}]