Which organism is useful in a biogas plant?

What does the given figure represent?

Methanogens are responsible for -

What is the chemical nature of biogas? Name an organism which is involved in biogas production.

Besides methane,which gas is produced in biogas?

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 $\xrightarrow{B}$ $C$)