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': 'Internal attachments for a partial denture is used: \nSelect and explain. \nA. When restoring a palatal cleft, B. When abutment tooth has dental canes, C. When there is difficult in getting clasp retention, D. When restoring Lost vertical dimension', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]