Which of the following gases is useful as an energy fuel?

Which gases are present in biogas?
$(i)$ Methane
$(ii)$ Hydrogen sulfide
$(iii)$ Carbon dioxide
$(iv)$ Carbon monoxide
$(v)$ Butane
$(vi)$ Ethane

During sewage treatment,the biogas produced includes:

Draw a diagrammatic sketch of a biogas plant and label its various components: Gas Holder,Sludge Chamber,Digester,and Dung + water chamber.

Which of the following compositions is called biogas?

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 $R$?