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
Difference between descriptive or analytical study and
experimental study is 
Select and explain. 
A. 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
D: All variables are under the control and can be modified by the operator
Answer: D: All variables are under the control and can be modified by the operator 
Explanation: None