The cleaning of water often happens in several steps involving Different techniques the cleaning process shown in the figure involves four steps in the second step the water is collected in a settling pond in what way does this step make the water cleaner. a) The bacteria in the water die.
b) oxygen is added to the water.
c) gravel and sand sink to the bottom
d) Toxic substances are broken down.

Water is cleaned for any impurities and is made safe for drinking, bathing, agriculture and many purposes. The water undergoes many processes to get treated and to get available for public use.

The second step makes the water cleaner by:

Option c) Gravel and sand sink to the bottom.

The process of water cleaning is explained as:

Therefore, in a second step, the gravel and sand sink at the bottom of the tank.

To learn more about techniques of water cleaning follow the link:

link

In the second step the gravel and sand sink to the bottom.

Explanation:

There are various techniques used for cleaning the water they are coagulation, sedimentation, filtration and disinfection.

Coagulation:

The process in which chemicals are added to water and the positive charge of the chemicals neutralizes the negative charge of dirt and other particles in water.

Sedimentation:

During this process the tiny mineral particles settle at the bottom of the water supply. This process of settling is known as sedimentation.

Filtration:

When the tiny particles has settled to the bottom of the water supply, the clear water will pass through the filters of varying compositions and pore sizes to remove the dissolved particles like dust, bacteria, viruses and chemicals.

Disinfection:

After filtration process a disinfectant like chlorine, chloromine are added to kill any remaining bacteria and viruses. The quality of water is tested and they are piped to all homes.

the awnser is d i think

geologic time scale (for gradpoint) =)