Which gas is produced in large amounts when bacteria act on cellulosic material in biogas production?

Statement $P$: Bacteria produce methane gas due to metabolic processes like anaerobic respiration.
Statement $Q$: Bacteria act on starchy materials.

Which bacteria are responsible for biogas production?

The first step in biogas production is carried out with the help of:

Study the following flow chart of biogas production and select the correct option for $A, B$ and $C$.
Stage $III$: $C$
Stage $II$: Organic acids $\xrightarrow{B}$ $C$
Stage $I$: Soluble compounds or monomers $\xrightarrow{A}$ Organic acids
(Note: The flow chart shows: Proteins,Fats,Cellulose,Hemicellulose $\rightarrow$ Soluble compounds or monomers $\xrightarrow{A}$ Organic acids $\xrightarrow{B}$ $C$)

Cattle dung used as a substrate in biogas production is a rich source of . . . . . . .