Direction: In the following questions, read the passage carefully and choose the best answer to each question out of the four alternatives.
PASSAGE
Biogas is a mixture of methane, carbon dioxide, hydrogen and hydrogen sulphide, the major constituent being methane. Biogas is produced by anaerobic degradation of animal wastes (sometimes plant wastes) in the presence of water. Anaerobic degradation means break–down of organic matter by bacteria in the absence of oxygen. Biogas is a non-polluting, clean and low-cost fuel which is very useful for rural areas where a lot of animal waste and agricultural waste are available. India has the largest cattle population in the world (240 million) and has tremendous potential for biogas production. From cattle dung alone, we can produce biogas of a magnitude of 22,500 mm 3 annually. Air-tight digestion/degradation of animal waste is safe as it eliminates health hazards which normally occur in case of direct use of dung due to direct exposure to faecal pathogens and parasites.