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
According to Schilder, mechanical objective of cleaning and shaping is/are: 
Select and explain. 
A. Canal should continuously taper, B. Canal should be prepared in multiple plane, C. Apical construction should be as small as possible, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: Schilder’s Mechanical Objectives of Cleaning and Shaping:

The root canal preparation should develop a continuously tapering cone.
Making the preparation in multiple planes which introduces the concept of “flow”.
Making the canal narrower apically and widest coronally.
Avoid transportation of the foramen.
Keep the apical opening as small as possible.