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': 'Incompetent lips refer to: \nSelect and explain. \nA. Inability of the lips to cover the incisors in the mandibular relaxed position, B. Inability of the lips to cover the incisors in occlusion, C. The lips come in between the upper and lower incisors, D. Tongue thrusts against the lips during swallowing', 'role': 'user'}, {'content': "Answer: A \nExplanation: The term 'incompetent lips' implies that the lips are unable to form an adequate seal under similar unstrained conditions, i.e.\xa0excessive separation of the lips at rest. As a general guideline, which holds for all ethnic groups, lip separation at rest should be no more than 3-4 mm; above this, the lips are termed incompetent.", 'role': 'assistant'}]