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 main ingredient in dental plaster is: 
Select and explain. 
A. Calcium sulphate hemihydrate, B. Calcium phosphate, C. Calcium anhydrate, D. Calcium sulphate dihydrate
A: Calcium sulphate hemihydrate
Answer: A: Calcium sulphate hemihydrate 
Explanation: None