This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Anatomy
What is the power of the lens used to view retina with this instrument? 
Select and explain. 
A. +20D, B. +30D, C. +15D, D. +90D
A: +20D
Answer: A: +20D 
Explanation: Indirect ophthalmoscopy needs usage of +20 D