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The answer I think is the environment for this one. <span />
Answer: It is because it makes it more efficient to study organisms.
Explanation: Biologist classify organisms by considering similarities between them. The organisms which have closely related characteristics are placed in a separate domains by biologists. Domains are further divided into 6 Kingdoms. According to modern classification system, domain is largest unit of biological classification.
Methanogens can be found in B.human intestines
Roots of plants and synthetic fertilizers can contain oxygen or come into contact with oxygen.