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: Dental
[{'content': 'Standard Deviation (SD) formula \nSelect and explain. \nA. Highest X-lowest X/2, B. Σ(X-X)/n, C. Σ(X-X)2/n, D. Σ(X-X)2/n-1', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]