Which organisms are responsible for digestion in a biogas plant?

What does the part labeled '$X$' in the given figure represent?

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

Assertion $A$: Herbivores release methane gas into the atmosphere.
Reason $R$: Herbivores consume a cellulose-rich diet.
Which option is correct for Assertion $A$ and Reason $R$?

Which gas is present predominantly in biogas?

Which type of carbohydrate,when processed by bacteria,yields the highest amount of methane?