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
Above which level of heat stress index it is not possible to work comfoably - 
Select and explain. 
A. 20 - 40, B. 40 - 60, C. 60 - 80, D. 80 - 100
B: 40 - 60
Answer: B: 40 - 60 
Explanation: 0 No thermal stress 10-30 Moderate to mild heat strain 40-60 severe heat strain 70-90 very severe heat strain 100 upper limit of heat tolerance Park's Textbook of Preventive and Social Medicine, 25th edition, Page No. 807