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: Medicine
Severity of spreading of plague is detected by 
Select and explain. 
A. Cheopis index, B. Total Flea index, C. Burrow's index, D. Chandler's index
A: Cheopis index
Answer: A: Cheopis index 
Explanation: Cheopis index:- It is average number of X. Cheopis / rat.