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
[{'content': 'Reversible pulpitis change to irreversible pulpitis primarily because of: \nSelect and explain. \nA. Vascular strangulation, B. Reduced host resistance, C. Invasion of microorganisms, D. An increase in microbial virulence', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]