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: Pharmacology
[{'content': 'All of the following are expectorants except \nSelect and explain. \nA. Hypertonic saline, B. Guaifenesin, C. Ammonium chloride, D. Noscapine', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Noscapine is a central acting antitussive, which is an opioid agonist.', 'role': 'assistant'}]