Correct option is D
Ratio of rate of back-washing to filtration in rapid sand filter
The process of back-washing is used to clean the filter bed after a certain period of time. During filtration, water passes through the filter bed and impurities are removed. The ratio of rate of back-washing to filtration is an important factor in determining the efficiency of the filter.
The ratio of rate of back-washing to filtration in a typical rapid sand filter is 10.
