Which gas is primarily produced in biogas plants?

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

Which of the following compositions is called biogas?

Which of the following represents the gas holder in the given biogas plant diagram?

Mohan was writing a report on the benefits of biogas:
$i$. It is cheap and safe
$ii$. Can be used for cooking
$iii$. Can be transported easily
$iv$. Causes pollution
Which one of the above statements has been reported incorrectly?

Which of the following is responsible for the production of gas during the decomposition of cow dung in a biogas plant?