b
(b) : Methanogens belong to archaebacteria. They include methane producing genera such as Methanobacillus and Methanothrix. Methanogens are obligate anaerobes found in oxygendeficient environments, such as marshes, swamps, sludge (formed during sewage treatment), and the digestive systems of ruminants. Mostly they obtain their energy by reducing carbon dioxide and oxidising hydrogen, with the production of methane.