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 length of the eyeball is – \nSelect and explain. \nA. 12mm, B. 16 mm, C. 20mm, D. 24 mm', 'role': 'user'}, {'content': 'Answer: D \nExplanation: The length of the eyeball is approximately 24 mm, So the distances from the cornea / crystalline lens to the retina and the focal length of optical rays focused by the cornea/crystalline lens propagating in the vitreous humor and imaging on the retina, are also about 24 mm.', 'role': 'assistant'}]