## Core Concept
The construction of a sanitary well involves several critical considerations to prevent contamination of the well water. One key aspect is the depth of the well lining, which is essential for ensuring the well's safety and water quality. The lining, often made of concrete rings, helps to stabilize the well and prevent surface contaminants from entering.

## Why the Correct Answer is Right
The correct answer, **20 feet**, is recommended because it helps to minimize the risk of contamination from surface water and shallow groundwater. At this depth, the risk of contamination from nearby sources such as surface water bodies, agricultural runoff, and shallow contaminated aquifers is significantly reduced. This depth also helps to ensure that the well is tapping into a more protected aquifer.

## Why Each Wrong Option is Incorrect
* **Option A: 5 feet** - This depth is too shallow and does not provide adequate protection against surface contaminants. Wells at this depth are highly susceptible to contamination from surface activities.
* **Option B: 10 feet** - Although slightly better than 5 feet, 10 feet is still relatively shallow. It may not be sufficient to protect against all types of contamination, especially in areas with shallow water tables or heavy agricultural use.
* **Option D: 30 feet** - While 30 feet provides even greater protection than 20 feet, the question specifically asks for the minimum depth. Therefore, 30 feet, although safer, is not the minimum required.

## Clinical Pearl / High-Yield Fact
A critical point to remember is that the construction and maintenance of sanitary wells are crucial for preventing waterborne diseases. The **20 feet** minimum depth for the lining is a general guideline and can vary depending on local regulations and hydrogeological conditions. Always consider local health department guidelines for specific requirements.

## Correct Answer: C. 20 feet