This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Systemic histology, Subject name: Anatomy
[{'content': 'Argentaffin cells in the GIT produce \nSelect and explain. \nA. Gastrin, B. Pepsin, C. HCl, D. None of the above', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Their granules contain a chemical called serotonin, which stimulates smooth muscle contractions.', 'role': 'assistant'}]