This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Stroke and TIA, Subject name: Medicine
[{'content': 'Below what values of cerebral blood flow, Ischemia occurs in the brain? \nSelect and explain. \nA. 20 mL/100 g/min, B. 30 mL/100 g/min, C. 40 mL/100 g/min, D. 50 mL/100 g/min', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Cerebral blood flow <20 mL/100 g tissue per minute cause Ischemia without infarction unless prolonged for several hours or days. Transient ischemic attack - Patient may experience only transient symptoms, if blood flow is restored to ischemic tissue before significant infarction develops. Normal cerebral blood flow (CBF) - 50-55 mL/100 g/min.', 'role': 'assistant'}]