This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Radiotherapy, Subject name: Radiology
A free radical is 
Select and explain. 
A. Any charged paicle, B. An atom or molecule with an unpaired electron in the outer shell, C. An atom with an even number of electrons, D. A chemically stable atom
B: An atom or molecule with an unpaired electron in the outer shell
Answer: B: An atom or molecule with an unpaired electron in the outer shell 
Explanation: Free radicals are atoms or groups of atoms with an odd (unpaired) number of electrons.Reference: Khan Physics of Radiation therapy; 5th edition