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': 'Which of the following represent the average distance of the fovea from the temporal margin of the optic disc? \nSelect and explain. \nA. 1 disc diameter, B. 2 disc diameter, C. 3 disc diameter, D. 4 disc diameter', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Fovea centralis is the central depressed pa of the macula. It is situated about 2 disc diameters (3mm) away from the temporal margin of the disc. It is about 1.5mm in diameter and is the most sensitive pa of the retina. It is tightly packed with cones and there are no rods. Ref: Comprehensive Ophthalmology By A K Khurana, 4th Edition, Pages 249, 251.', 'role': 'assistant'}]