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
The zinc oxide-eugenol impression pastes harden by: 
Select and explain. 
A. Chemical reaction, B. Cold, C. Heat, D. Pressure
A: Chemical reaction
Answer: A: Chemical reaction 
Explanation: Key Concept:  ZnO Eugenol impression paste hardens by chemical reaction.