Air does not ordinarily contain: Select one:

a. carbon dioxide

b. water vapor

c. oxygen

d. nitrogen-fixing bacteria

The correct answer is nitrogen-fixing bacteria.

Nitrogen-fixing bacteria refer to the microbes with the tendency of converting atmospheric nitrogen into fixed nitrogen, that is, inorganic components usable by the plants. About more than 90 percent of the fixation of nitrogen is influenced by these species, thus, they play an essential function in the nitrogen cycle.

Two kinds of nitrogen-fixing bacteria are identified. These are nonsymbiotic or free-living bacteria, which includes Anabaena, Nostoc, and Cyanobacteria. The second one comprises of symbiotic or mutualistic bacteria, this includes Rhizobium, Frankia, and Azosprillum species.