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
Study models are used: 
Select and explain. 
A. As references in orthodontic cases, B. To show shape, size and position of teeth, C. As an aid in treatment planning, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: None