Methanogens,growing anaerobically on cellulosic material,produce

The type of gases produced in a biogas plant depends on which of the following?

Which of the following represents the gas holder in the given biogas plant diagram?

Which gas is primarily produced in biogas plants?

Which type of bacteria is present in the rumen (a part of the stomach) of cattle?

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?