The process of purifying water by spraying water on to a bed of stones where microorganism live so that the microorganisms feed on the pathogens in water and make the water free from harmful pathogens is called?

[A] Stone bed method
[B] biofilter method
[C] aerobic filtration
[D] anerobic filtration

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This question is part of UPSC Prelims Practice and Mock Tests membership programme.