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: Dental
Heat hardening is the process done at 
Select and explain. 
A. Heating 750°C and slowly cooling to zero degrees, B. Heating quickly upto 450°C followed by sudden quenching, C. Heating upto 250°C for 15 to 30 min, D. Heating at 750° C and then quenching to room temperature
C: Heating upto 250°C for 15 to 30 min
Answer: C: Heating upto 250°C for 15 to 30 min 
Explanation: None