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': 'Blood flow through a vessel varies directly with \nSelect and explain. \nA. Resistance, B. Viscosity, C. Difference of pressure, D. Length of vessel', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Blood flow is directly proportional to\n-\xa0\xa0\xa0\xa0\xa0\xa0\xa0 Pressure gradient\n-\xa0\xa0\xa0\xa0\xa0\xa0\xa0 Radius\nBlood flow is inversely proportional to\n-\xa0\xa0\xa0\xa0\xa0\xa0\xa0 Viscosity\n-\xa0\xa0\xa0\xa0\xa0\xa0\xa0 Length\n-\xa0\xa0\xa0\xa0\xa0\xa0\xa0 Resistance', 'role': 'assistant'}]