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: Social & Preventive Medicine
Dichloro-diphenyl-trichloroethane (DDT) is one of the most widely used insecticide. Which of the following statement is NOT true about DDT? 
Select and explain. 
A. Pyrethrum has synergistic action, B. It is a contact poison, C. Immediately kills the prey, D. Residual effect lasts for 18 months
C: Immediately kills the prey
Answer: C: Immediately kills the prey 
Explanation: Dichloro-diphenyl-trichloroethane (DDT) is primarily a contact poison. It acts on nervous system of insects. It permeates into the insect body through the cuticle, after dissolving in the waxy covering of the feet, and causes of paralysis of legs and wings, convulsions and finally death. DDT does not immediately kill the insects. It takes several hours to kill them. Ref: Park's Textbook of Preventive and Social medicine, 19th edition, Page 637.