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Solution:

The correct option is (B).
After the BOD of sewage or waste water is reduced significantly, the effluent is passed into a settling tank where the bacterial ‘flocs’ are allowed to sediment. This sediment is called activated sludge. A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. The activated sludge contains about 97% water. This water is removed from the sludge by using sand drying beds or machines. 
 

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