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 a type of water - treatment device. It is typically a concrete or plastic box filled with layers of sand and gravel which help in filtering water through physical and biological processes.

Answer:

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