Microbes in Human Welfare

Biogas generation is a three stage anaerobic digestion of animal and other organic wastes. Study the following flow chart and select the correct option for stages I, II and Ill. 
Which one is a biofertilizer?
Which of the following statements is correct with regard to biocontrol agents ?
Which of the following is used as a green manure?
The diagram is that of a typical biogas plant. Explain the sequence of events occurring in a biogas plant. Identify $$a, b, c$$