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': 'Carry over effect is seen in \nSelect and explain. \nA. Case control study, B. Concurrent parallel trial, C. Cross over study, D. Cohort study', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]