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
The following are all residual insecticides except – 
Select and explain. 
A. Pyrethrum, B. DDT, C. Gamma BHC, D. Malathion
A: Pyrethrum
Answer: A: Pyrethrum 
Explanation: None