This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Elementary Optics, Subject name: Ophthalmology
Below is a common instrument used in ophthalmology. In order to view the entire retina what is the power of lens you would like to attach to the instrument 
Select and explain. 
A. +90 D, B. +78 D, C. -58 D, D. +20 D
D: +20 D
Answer: D: +20 D 
Explanation: Ans. (d) +20 DRef: Yanoff & Ducker 4/e, p. 89*Instrument is indirect ophthalmoscope.*Condensing lens commonly used is 20D biconvex lens.#Magnification=PoweroflensPowerofeye =20D60D =3times