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 difference in the colour between sub and supragingival calculus is related to? 
Select and explain. 
A. pH of plaque, B. Death of leucocytes, C. Hemotysis of erythrocytes, D. All of the above
C: Hemotysis of erythrocytes
Answer: C: Hemotysis of erythrocytes 
Explanation: None