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
[{'content': 'Radiation hazard is absent in -a) MRIb) Doppler USGc) Digital substraction angiographyd) Tc 99 scan \nSelect and explain. \nA. ab, B. a, C. ad, D. bc', 'role': 'user'}, {'content': 'Answer: A \nExplanation: USG and MRI have no radiation exposure.', 'role': 'assistant'}]