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
Agar impression materials differ from alginate impression materials in that the former sets by: 
Select and explain. 
A. Mechanical action of saliva, B. Physical change, C. Evolution, D. Chemical change
B: Physical change
Answer: B: Physical change 
Explanation: None