This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
Effect of Pyrethrum on mosquitoes is described as - 
Select and explain. 
A. Residual, B. Repellent, C. Knock down, D. Contact
D: Contact
Answer: D: Contact 
Explanation: Pyrethrum extract is an excellent space spray. the active principle(pyrethrin) is a nerve poison and kills insect instantly on mere contact. Sprayed at a dose of 1 Oz of the spray solution per 1000 Cu.ft.of space. Doors and windows are kept closed for half an hour. Effective in reducing the number of mosquitoes but the reaction is only temporary since it has no residual action. Park's Textbook of Preventive and Social Medicine, 25th Edition, Pg 833