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.

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.