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
Impression material that do not harden by chemical reaction: 
Select and explain. 
A. Zinc oxide eugenol, B. Impression compound, C. Alginate, D. Plaster of paris
B: Impression compound
Answer: B: Impression compound 
Explanation: Reversible materials, such as agar and impression compound, soften upon heating and solidify slightly above body temperature with no chemical change taking place.