This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory system, Subject name: Medicine
[{'content': 'Cyanosis cannot occur in severe anaemia because \nSelect and explain. \nA. Anaemic blood has a higher O2 carrying capacity per gram of Hb, B. It requires a critical concentration of reduced Hb in blood, C. Patient improves his alveolar oxygen as a compensation for anaemia, D. There is an increased blood flow through the skin', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Cyanosis seen when deoxygenated hb >4 gm. so in severe anemia where HB less than 4 gm we cannot see cyanosis.', 'role': 'assistant'}]