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: Biochemistry
[{'content': 'Free radicals are chemical species which have \nSelect and explain. \nA. Single unpaired electron in an outer orbit, B. Single paired electron in an outer orbit, C. Double unpaired electron in an outer orbit, D. Double paired electron in an outer orbit', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]