The correct answer is A. 10 to 25.
Plain sedimentation is a physical process that removes suspended solids from water by gravity. The solids settle to the bottom of the tank, where they can be removed. Plain sedimentation is a relatively simple and inexpensive process, but it is not very effective at removing bacteria.
Bacteria are very small and can remain suspended in water for long periods of time. They are also able to attach to particles in the water, which makes them more difficult to remove. As a result, plain sedimentation can only remove a small percentage of the bacterial load from water.
To remove more bacteria from water, other treatment methods, such as filtration or disinfection, are necessary.
Option B is incorrect because plain sedimentation is not very effective at removing bacteria.
Option C is incorrect because plain sedimentation is not able to remove all of the bacteria from water.
Option D is incorrect because plain sedimentation is not a 100% effective process.