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: Ophthalmology
[{'content': 'The critical angle refers to the angle of incidence in the denser medium, corresponding to which angle of refraction in the rarer medium is: \nSelect and explain. \nA. Less than 90deg, B. 90deg, C. More than 90deg, D. Equal to angle of incidence', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. 90deg', 'role': 'assistant'}]