This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Ophthalmology
[{'content': 'Astigmatism is considered to be: \nSelect and explain. \nA. Spherical abberation, B. Curvatural ametropia, C. Axial ametropia, D. Index ametropia', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. Spherical abberation', 'role': 'assistant'}]