This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Communicable diseases, Subject name: Social & Preventive Medicine
A malarial survey is conducted in 50 villages having a population of one lakh. Out of 20000 slides examined, 500 turned out to be malaria positive. The annual parasite incidence is - 
Select and explain. 
A. 20%, B. 5%, C. 0.50%, D. 0.40%
C: 0.50%
Answer: C: 0.50% 
Explanation: <> -Annual parasite incidence. = confirmed cases during one year *1000 ---------------------------------------------------------- Population under surveillance (0.5%) -It is based on intensive active and passive surveillance and cases are confirmed by blood examination. Reference : Park's textbook of preventive and social medicine, 23rd edition, pg no:261 <\>