"nitrifiers" use two steps to convert ___ to ___.

• A. ammonia - nitrate
• B. nitrate - nitrite
• C. nitrite - nitrate
• D. ammonia - nitrite

Answer: (A)

Nitrification is an aerobic process in which inorganic nitrogen is oxidized in two steps. First, ammonia (NH3/NH4+) is oxidized to nitrite (NO2−); then nitrite is oxidized to nitrate (NO3−). The overall change is ammonia to nitrate, which is why describing the two-step sequence as converting ammonia to nitrate best fits the process. The intermediate nitrite is produced and then consumed in the second step, carried out by different nitrifiers (ammonia-oxidizing organisms first, nitrite-oxidizing organisms second). The other wording would misstate the direction or omit the intermediate step, so the ammonia to nitrate framing accurately reflects how nitrification proceeds.