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': 'Modifiers are added to gypsum mainly to \nSelect and explain. \nA. Modify setting time, B. Modify setting expansion, C. Modify strength, D. Decrease the porosity', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]