In sewage-treatment systems, nitrifiers such as ________ are used to decrease the reduced nitrogen content.

A) Neisseria gonorrhoeae and N. meningitidis
B) Thiobacillus spp.
C) Nitrosomonas, Nitrosolobus, and Nitrosovibrio
D) Escherichia spp.
E) Helicobacter and Campylobacter