Only Certain Prokaryotes Can Perform Nitrogen Fixation, but Nitrogen-Fixing Prokaryotes

Multiple Choice

Only certain prokaryotes can perform nitrogen fixation, but nitrogen-fixing prokaryotes are not known to live inside animals. Thus, how do animals gain access to fixed nitrogen?

A) They may breathe it in from air that has experienced lightning discharges.
B) They may ingest nitrogen fixers.
C) They may ingest plants that harbor nitrogen fixers, or plants that absorbed fixed nitrogen from the soil.
D) They may ingest other animals that had done either (B) or (C) above.
E) Answers (B) , (C) , and (D) above are all possible.