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
[{'content': 'Phosphate bonded investment is made carbon free \nSelect and explain. \nA. To prevent brittleness of alloy., B. To produce smooth casting., C. To prevent staining  of ceramic., D. For easier divesting of investment.', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]