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
Snellen's chart is based on 
Select and explain. 
A. Light sense, B. Colour sense, C. Contrast sense, D. Form sense
D: Form sense
Answer: D: Form sense 
Explanation: Snellen's chart is based on Form sense.