Question

the waterworks uses this to make sure all of the small pieces of dirt are out of the water;
a strainer
bleach
gravel
a net

Explanation:

To remove small dirt particles from water, a strainer is used in waterworks for filtration (straining out small solids). Bleach is for disinfection, gravel is part of filtration systems but not the tool to remove small dirt directly, and a net is for larger debris. So the correct option is the one with "a strainer".

Answer:

a. a strainer