CBSE Class 10 Science 2018 · Paper 31(B) · Question 15

(a) Methane (${\text{CH}}_4$) is the major fuel component of biogas; it makes up to about $75\%$ of the gas. (b) Two advantages of biogas over wood or fossil fuels: (i) it burns without smoke, so it causes little pollution; (ii) it has a high calorific value and leaves no residue such as ash. (Other correct advantages, such as the spent slurry being a good manure, also earn credit.) (c) Two values shown by the sarpanch: (i) concern for cleanliness and the environment of the village, including saving trees; (ii) a sense of togetherness and responsibility, in seeking advice and getting the villagers to work collectively. Awareness about non-conventional sources of energy is another acceptable value.