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 normal axial length of the eye - 
Select and explain. 
A. 18mm, B. 20 mm, C. 28 mm, D. 24mm
D: 24mm
Answer: D: 24mm 
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 camera/crystalline lens propagating in the vitreous humor and imaging on the retina, are also about 24 mm