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': 'Agar impression materials differ from alginate impression materials in that the former sets by: \nSelect and explain. \nA. Mechanical action of saliva, B. Physical change, C. Evolution, D. Chemical change', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]