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
[{'content': 'Duochrome test is used for \nSelect and explain. \nA. Colour blindness, B. Cylindrical axis, C. Spherical axis, D. Astigmatism', 'role': 'user'}, {'content': 'Answer: C \nExplanation: 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', 'role': 'assistant'}]