Before denitrification can take place, there must be presence of food (BOD), absence of oxygen, and presence of nitrate.

• A. Presence of food (BOD), absence of oxygen, and presence of nitrate.
• B. Absence of food (BOD), presence of oxygen, and absence of nitrate.
• C. Absence of oxygen, absence of nitrate, and presence of BOD.
• D. Presence of nitrate, presence of oxygen, and absence of BOD.

Answer: (A)