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': 'Thinnest layer of tear film \nSelect and explain. \nA. Mucus layer, B. Aqueous layer, C. Lipid layer, D. Oily layer', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Thickest - Aqueous layer.\nThinnest - Mucus layer.', 'role': 'assistant'}]