This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Optics and refraction, Subject name: Ophthalmology
Duochrome test is used for 
Select and explain. 
A. Colour blindness, B. Cylindrical axis, C. Spherical axis, D. Astigmatism
C: Spherical axis
Answer: C: Spherical axis 
Explanation: Refining the sphere : The spherical correction is refined after refining the cylinder power and axis . It is done by following tests : Fogging technique Duochrome test Pin-hole test Reference : A.K.Khurana; 6th edition; Page no: 578