Which is the main gas produced in biogas?

Which bacteria is utilized in a Gobar gas plant?

Where are methanogens found?

Consider the following statements about methanogenic bacteria:
$I.$ Methanogenic bacteria are commonly found in the anaerobic sludge formed during sewage treatment.
$II.$ These bacteria also occur in the rumen of cattle,where they act upon cellulosic material to break down cellulose.
$III.$ They play a very important role in the nutrition of cattle by digesting cellulosic material.
Which of the statements given above are correct?

$A$: Biogas is a mixture of various gases.
$R$: The type of gases produced depends on the microbes and the organic substrate used.

Which substance remains undigested during the anaerobic respiration of biological waste in the biogas production process?