This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
[{'content': 'At which level of heat stress index it is not possible to work comfoably causing threat to health: \nSelect and explain. \nA. 20-40, B. 40-60, C. 60-80, D. 80-100', 'role': 'user'}, {'content': 'Answer: B \nExplanation: HEAT STRESS INDEX(HSI): HSI% Consequence of 8 hour exposure 0 No thermal strain 10-30 Mild-Moderate heat stress, Minimal impairement in work 40-60 Severe heat stress, Threat to health if not fit 70-90 Very severe heat stress, only few can sustain it 100 Maximum heat stress, only young fit acclimatized can sustain it >100 Varying degrees of stress due to hypehermia Ref: Park 25th edition Pgno: 784', 'role': 'assistant'}]