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': 'Difference between descriptive or analytical study and\nexperimental study is \nSelect and explain. \nA. Descriptive study has a small sample, B. Experimental study requires a strong knowledge of statistics by the operator, C. Analytical study requires the use of sophisticated instruments, D. All variables are under the control and can be modified by the operator', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]