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': 'Back pressure porosity can be avoided by \nSelect and explain. \nA. Using asbestos liners, B. Placing the sprue at least one quarter inch away from the end of casting ring, C. Preventing rapid heating of investment, D. Using a short and wide sprue', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]