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
Casting flux is used to 
Select and explain. 
A. Prevent the oxidation of material during melting., B. Increase the melting point of the flux., C. Prevent the contamination of the metal and theliner., D. All of the above.
A: Prevent the oxidation of material during melting.
Answer: A: Prevent the oxidation of material during melting. 
Explanation: Fluxes are specifically used

To remove gas or prevent its absorption into the melt.
To reduce metal loss.
To remove specific impurities and nonmetallic inclusions
To refine metallic constituents, or to lubricate and control surface structure in the semi-continuous casting of metal alloys.

Key concept
Flux powder may be sprinkled over the molten metal to reduce the oxides and increase fluidity for casting.