When does the $BOD$ level in effluent decrease?

• A
  The amount of $BOD$ decreases when organic matter is removed as sludge.
• B
  The $BOD$ level decreases because bacteria and other microorganisms consume the majority of the organic matter present in the water.
• C
  The $BOD$ level can be reduced by oxidizing and reducing a large amount of inorganic matter in the water.
• D
  The $BOD$ level can be reduced by first oxidizing the organic and inorganic matter in the effluent and then adding $CO_2$.