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
Pickling is done: 
Select and explain. 
A. To remove oxide film from casting, B. Polish the casting, C. Improve the strength of casting, D. Avoid casting defects
A: To remove oxide film from casting
Answer: A: To remove oxide film from casting 
Explanation: None