31. Nitrogen fixation

A. changes the free nitrogen (N₂) to a form usable by plants
B. especially changes nitrogen compounds, mostly amines such as NH₂
C. both (a) and (b)
D. fix the free nitrogen (N₂) by which it should not be usable by plants

32. For rapid decomposition by microbes, the substrate should have a C/N ratio of

A. 10-20
B. 20-30
C. 30-40
D. 60-80

33. Which are the main source of biofertilisers?

A. Cyanobacteria
B. Bacillus
C. Streptococcus
D. None of these

34. The organisms responsible for the characteristic musty or earth odor of a freshly plowed field is/are

A. Nocardia
B. Streptomyces
C. Micromonospora
D. all of these

35. Denitrification is

A. reduction of nitrate (NO₃^–) to nitrogen gas
B. reduction of nitrate to organic nitrogen compounds
C. both (a) and (b)
D. changing of atmospheric nitrogen (N₂) to nitrogen compounds