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': 'Suckback porosity is due to: \nSelect and explain. \nA. Attached impurities on undersurface of casting, B. Hot spot, C. No rapid cooling, D. Inadequate melting temperature', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Suck-back porosity often occurs at an occlusoaxial line angle or incisoaxial line angle that is not well rounded.\xa0\nA hot spot may retain a localized pool of molten metal after other areas of the casting have solidified. This, in turn, creates a shrinkage void or suck-back porosity.\xa0\nThis type of porosity defect can be eliminated by flaring the point of sprue attachment and reducing the mold-melt\xa0temperature differential—that is, lowering the casting temperature by about 30°C.\n\nReference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, 12th ed page no 227', 'role': 'assistant'}]