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': "In Angle's classification for malocclusion, key tooth is: \nSelect and explain. \nA. MB cusp of Permanent Max. 1st molar, B. DB cusp of Permanent Max. 1st molar, C. MB cusp of Permanent Max. 2nd molar, D. MB cusp of Permanent Mand. 1st molar", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Angle’s postulate was that the upper first molars\xa0 were the key to occlusion and that the upper and lower molars\xa0 should\xa0 be\xa0 related\xa0 so\xa0 that\xa0 the\xa0 mesiobuccal\xa0 cusp\xa0 of\xa0 the\xa0 upper molar occludes in the buccal groove of the lower molar. If the teeth were arranged on a smoothly curving line of occlusion and this molar relationship existed , then normal occlusion\xa0 would result.\xa0\n\xa0\nAngle then described three classes of malocclusion, based on the occlusal relationships of the first molars:\n\nClass I: Normal relationship of the molars, but line of occlusion incorrect because of malposed teeth, rotations, or other causes.\nClass II: Lower molar distally\xa0 positioned relative to upper molar, line of occlusion not specified.\nClass III: Lower molar mesially positioned relative to upper molar, line of occlusion not specified.\n\nReference: Contemporary orthodontics, Proffit 6th ed page no 2', 'role': 'assistant'}]