**Core Concept**
Slow sand filtration is a traditional method for water treatment, utilizing the natural processes of sedimentation, filtration, and biological activity to remove contaminants and improve water quality. This method relies on the presence of a specific layer of sand, which supports the growth of beneficial microorganisms that help break down organic matter and pathogens.

**Why the Correct Answer is Right**
The correct answer highlights the key characteristic of slow sand filtration: it requires a longer duration to effectively remove contaminants and improve water quality. This is because the sand layer needs time to support the growth of beneficial microorganisms, which are responsible for breaking down organic matter and pathogens. The slow sand filtration process typically involves a flow rate of 0.1-0.2 meters per hour, allowing for sufficient contact time between the water and the sand layer. As a result, slow sand filtration requires a longer duration, typically ranging from 24 to 48 hours, to achieve the desired level of water treatment.

**Why Each Wrong Option is Incorrect**
* **Option A:** Slow sand filtration actually occupies more space due to the need for a larger filter bed to accommodate the sand layer and support the growth of beneficial microorganisms.
* **Option B:** While slow sand filtration may be more labor-intensive and require more maintenance than other water treatment methods, it is not necessarily more expensive. In fact, slow sand filtration can be a cost-effective option for small-scale water treatment applications.
* **Option D:** The optimal sand size for slow sand filtration is typically between 0.15 and 0.35 millimeters, which allows for the effective removal of contaminants and supports the growth of beneficial microorganisms. A sand size of 0.4-0.7 mm is too large and would not be effective for slow sand filtration.

**Clinical Pearl / High-Yield Fact**
Slow sand filtration is a natural and effective method for removing contaminants and improving water quality. It is often used in combination with other water treatment methods to provide a comprehensive solution for water treatment. By understanding the principles of slow sand filtration, public health professionals can design and implement effective water treatment systems that meet the needs of communities around the world.

**✓ Correct Answer: C. Requires longer duration**