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': 'Retention in a composite restoration is mainly attained by \nSelect and explain. \nA. Chemical adhesion, B. Acid etching & Bonding, C. Inverted Truncated shape, D. Undercuts', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]