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': 'The recommended rubber dam to provide effective retraction of gingival tissue is \nSelect and explain. \nA. Thin, B. Extra heavy, C. Heavy, D. Medium', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]