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': 'Precapillary sphincter relaxation is caused by \nSelect and explain. \nA. Sympathetic activity, B. Circulating catecholamines, C. Local metabolites, D. Fall in capillary pressure', 'role': 'user'}, {'content': 'Answer: C \nExplanation: There is no autonomic supply to precapillary sphincters but control of these sphincters is done by oxygen concentration in tissue and local metabolites.', 'role': 'assistant'}]