This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Special Senses, Subject name: Physiology
For detection of light, minimum amount of light (quanta) reaching retina should be: 
Select and explain. 
A. 1, B. 6-May, C. 54, D. More than 500
B: 6-May
Answer: B: 6-May 
Explanation: "Visual threshold" is the minimum amount of light detected by retina. Threshold for a single rod is 1 quantum of light. To distinguish light from this discharge, at least 5-6 rods should be excited synchronously. Hence, about 5-6 quanta of light is the 'visual threshold' for retina. For 5-6 rods to be excited, about 100 quanta of light should reach the eye (cornea). Many of the quanta are absorbed, reflected, or refracted in the intervening ocular media.