Methanogens are found in
$I.$ organic acid
$II.$ rumen of cattle
$III.$ butanal
$IV.$ anaerobic sludge
Choose the correct option

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?

Cow dung is appropriately used as

In a biogas plant,when does the gas holder (cover) rise?

During biogas production,acetic acid is transformed into the final product by the enzymes of:

In the given diagram of a biogas plant,what do '$Q$' and '$S$' represent?