Which of the above statements are true?
B is the correct answer. I and III are basic: denitrification is anaerobic and is the reduction of NO3 to N2. V is a little trickier, the pathway for denitrification is NO3--NO2--N2. As for II, many species are capable of denitrification, not just a few. Which ones are present in a wastewater plant depends on the plant. As for IV, all receiving waters have some capacity to handle a nitrate load without adversely effecting the ecosystem. If that capacity is still unmet, denitrification offers no advantages.