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': 'Subsurface porosity is due to \nSelect and explain. \nA. Thermal changes, B. Thickness of the resin, C. Lack of temperature, D. Packing past the dough stage', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]