Biogas is composed of .......

Choose the correct sentence.

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

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?

Identify the correct statements from the following:

In $STPs$,in which tank is biogas produced during the process?