Question

what is a biosand filter?
a concrete or plastic box that is filled with layers of sand and gravel
a concrete or plastic box that is filled with active carbon and chemicals
a paper box that is filled with rice husks or coffee husks
a paper box that is filled with different minerals

Explanation:

A biosand filter is typically a concrete or plastic box filled with layers of sand and gravel which aid in water - purification through physical and biological processes. The other options describe different types of containers and fillings that are not characteristic of a biosand filter.

Answer:

A. A concrete or plastic box that is filled with layers of sand and gravel