Which of the following gases is released during the decomposition of organic matter?

The diagrams above are the illustrations of organisms from different habitat. Study them and answer questions 32 and 33.

The structures labelled I are for 

Habitats of the organisms respectively are 

An environmental problem caused by Sulphur (IV) oxide is 

The phenomenon by which a farmland that is left to fallow for several years develops into a forest is called