Explanation:

Statement (b) is not correct.
Biogas refers to a gas produced by the biological breakdown of organic matter in the absence of oxygen. It is a type of biofuel. It is produced by anaerobic fermentation of bio-degradable materials such as biomass, manure or sewage, municipal waste, green waste and energy crops. Importance of biogas:

• When cow dung is put in a biogas plant, two main products are obtained- (a) methane gas which can be used as a fuel for domestic purposes and (b) liquid humus that can be used as a natural fertilizer.
• Biogas can be generated from locally available materials like animal dung, vegetable wastes, agricultural wastes, etc. In this way, waste is a managed and converted into useful products.
• Biogas is a clean fuel as it burns without smoke or any odour and is ashless.
• Biogas has a high calorific value. It has been estimated that a small family of four persons required 4.25% of biogas per day.
• Biogas provides about 5 times more energy than the direct burning of the same quantity of dried dung cakes.
• The fertilizer left after extracting biogas has 43% more strength than dung used directly as manure