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: Physiology
[{'content': 'Surfactant lining the alveoli \nSelect and explain. \nA. Helps prevent alveolar collapse., B. Is produced in alveolar type I cells and secreted into the alveolus., C. Is increased in the lungs of heavy smokers., D. Is a glycolipid complex.', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The low surface tension when the alveoli are small is due to the presence in the fluid lining the alveoli of surfactant, a lipid surface-tension-lowering agent. If the surface tension is not kept low when the alveoli become smaller during expiration, they collapse in accordance with the law of Laplace.', 'role': 'assistant'}]