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 length of the eyeball is – 
Select and explain. 
A. 12mm, B. 16 mm, C. 20mm, D. 24 mm
D: 24 mm
Answer: D: 24 mm 
Explanation: 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.