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 standard power of an intraocular lens implanted in the posterior chamber is: \nSelect and explain. \nA. 18 dioptres, B. 20 dioptres, C. 23 dioptres, D. 25 dioptres', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. 20 dioptres', 'role': 'assistant'}]