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: Dental
[{'content': 'Which of the following is not true of polymethylmethacrylate? \nSelect and explain. \nA. Knoop hardness number 18 to 20, B. Tensile strength of 60 MPa, C. Density of 1.13 g/cm3, D. Modulus of elasticity of 3.4 GPa', 'role': 'user'}, {'content': 'Answer: D \nExplanation: PMMA is a transparent resin of water-like clarity that is transparent to light in the visible and ultraviolet range down to a wavelength of 250 nm. It is a hard resin with a Knoop hardness number of 18 to 20 KHN. It has a tensile strength approximately 60 MPa, a density of 1.19 g/cm3\xa0and a modulus of elasticity of approximately 2.4 GPa.\xa0\nIt is also extremely stable: it does not discolor in ultraviolet light and it exhibits remarkable aging properties. It is chemically stable to heat below 125°C, softens at 125°C, and can be molded as a thermoplastic material. However, above 125°C, PMMA begins to depolymerize and forms methylmethacrylate monomer, MMA, and at approximately 450°C, 90% of the polymer will have depolymerized.\nReference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, 12th ed page no 108', 'role': 'assistant'}]