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
The critical angle refers to the angle of incidence in the denser medium, corresponding to which angle of refraction in the rarer medium is: 
Select and explain. 
A. Less than 90deg, B. 90deg, C. More than 90deg, D. Equal to angle of incidence
B: 90deg
Answer: B: 90deg 
Explanation: Ans. 90deg