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
Radiation hazard is absent in -a) MRIb) Doppler USGc) Digital substraction angiographyd) Tc 99 scan 
Select and explain. 
A. ab, B. a, C. ad, D. bc
A: ab
Answer: A: ab 
Explanation: USG and MRI have no radiation exposure.