**Insecticides and their Classification**
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### Core Concept

Insecticides are chemicals used to control or eliminate insect pests. They can be classified into several categories, including residual and non-residual (or contact) insecticides. Residual insecticides remain effective against insects for a period of time after application, often through a process known as systemic toxicity.

### Why the Correct Answer is Right

Residual insecticides, such as organophosphates, carbamates, and pyrethroids, work by interfering with the insect's nervous system or by disrupting their metabolism. These chemicals can be absorbed by plants and animals, allowing them to exert their toxic effects over time. The most common example of a residual insecticide is probably DDT, which was widely used in the past but has largely been phased out due to environmental and health concerns.

### Why Each Wrong Option is Incorrect

**Option A:** This option is not provided.
**Option B:** This option is not provided.
**Option C:** This option is not provided.

### Clinical Pearl / High-Yield Fact

One important consideration when using residual insecticides is their potential to accumulate in the environment and cause harm to non-target species, including humans. It's essential to use these chemicals judiciously and follow proper application and disposal protocols to minimize risks.

### Correct Answer: D. Malathion