Concept:Denitrifying bacteria convert nitrates in the soil into gaseous nitrogen, which is released into the atmosphere.Explanation:Nitrifying bacteria convert ammonia into nitrates.Denitrifying bacteria reduce nitrates to gaseous nitrogen, completing the nitrogen cycle.Putrefying and saprophytic bacteria decompose dead organic matter.Thus, the bacteria that reduce nitrates to gaseous nitrogen are denitrifying bacteria.Answer:B. denitrifying bacteria