This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Ultrasonography, CT, and MRI, Subject name: Radiology
MRI utilizes - 
Select and explain. 
A. Hydrogen atoms, B. X-rays, C. Helium ion, D. Radioactivity
A: Hydrogen atoms
Answer: A: Hydrogen atoms 
Explanation: Ans. is 'a' i.e., Hydrogen atoms o MRI is a technique that produces tomographic images by means of magnetic fields and radiofrequency waves.o It causes some alignment of the hydrogen atoms within the body along the magnetic field.o MRI is based on the ability of a small number of protons within the body to absorb and emit radio wave energy when the body is placed within a strong magnetic field.o MR scans are obtained by placing the patient in a static magnetic field of 0.02 to 4 tesla (T) Strength