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 the investment in burnout oven, D. Using a short and wide sprue', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Backpressure porosity\xa0\nThis is caused by inadequate venting of the mold. Air is trapped in the mold and is unable to escape. The sprue pattern length should be adjusted so that there is not more than 1/4" thickness of the investment between the bottom of the casting ring and the wax pattern. When the molten metal enters the mold, the air inside is pushed out through the porous investment at the bottom. If the bulk of the investment is too great, the escape of air becomes difficult causing increased pressure in the mold. The gold will then solidify before the mold is completely filled resulting in a porous casting with rounded short margins.\xa0\nAvoided by:\n\nUsing adequate casting force.\xa0\nUse investment of adequate porosity.\xa0\nPlace pattern not more than 6 to 8 mm away from the end of the ring.\nProviding vents in large castings.\xa0\n\nManapallil 3rd P:370', 'role': 'assistant'}]