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: Anatomy
[{'content': 'Single unit smooth muscles are seen in \nSelect and explain. \nA. Iris, B. Ductus deferens, C. Ureter, D. Trachea', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]