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: Pathology
Reversible pulpitis change to irreversible pulpitis primarily because of: 
Select and explain. 
A. Vascular strangulation, B. Reduced host resistance, C. Invasion of microorganisms, D. An increase in microbial virulence
C: Invasion of microorganisms
Answer: C: Invasion of microorganisms 
Explanation: None