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': 'The focal length of 0.25 m power of the lens is \nSelect and explain. \nA. 40 D, B. 4 D, C. 1/4 D, D. 25 D', 'role': 'user'}, {'content': 'Answer: B \nExplanation: It is measured\xa0as reciprocal of the focal length in metres i.e.\xa0P\xa0= 1/f .\xa0\xa0The unit of power is dioptre (D). One\xa0dioptre is the power of a lens of focal length one\xa0metre.\n\nThe focal length (f) of a lens is the distance\xa0between the optical centre and the principal focus.\nPower of a lens (P) is defined as the ability of the\xa0lens to converge a beam of light falling on the\xa0lens.', 'role': 'assistant'}]