**Core Concept**
The Nalgonda technique is a method used for defluoridation of water, which involves the removal of excess fluoride from drinking water to prevent dental and skeletal fluorosis. This technique employs a combination of lime and alum to precipitate out the fluoride ions.

**Why the Correct Answer is Right**
The Nalgonda technique involves the addition of lime (calcium hydroxide) and alum (aluminum sulfate) to the water. The lime reacts with the fluoride ions to form a solid precipitate of calcium fluoride, which can be easily removed from the water. The alum helps to coagulate the impurities and precipitates, making it easier to remove the fluoride ions. The sequence of addition is crucial, as the alum is added after the lime to ensure effective precipitation of the fluoride ions.

**Why Each Wrong Option is Incorrect**
* **Option A:** The sequence of addition of lime and alum is incorrect. Lime should be added before alum.
* **Option B:** The use of lime and alum is not the only method for defluoridation of water. Other methods, such as activated alumina, can also be used.
* **Option C:** The sequence of addition of lime and alum is not specified in this option.

**Clinical Pearl / High-Yield Fact**
The Nalgonda technique is a simple and cost-effective method for defluoridation of water, making it an ideal solution for communities with high levels of fluoride in their drinking water.

**Correct Answer: A. Lime followed by alum.**