(a) The vegetation gets submerged under the waters, in a dam. It rots under anaerobic conditions and produces methane, which is a greenhouse gas.

(b) Technological inputs to improve the efficiency of bio mass fuels are:-

(i) Burning wood in a limited supply of oxygen. The water and volatile materials present in the wood get removed and charcoal is left behind.

Charcoal burns flames and is relatively smokeless and with higher heating efficiency.

(ii) Similarly, cow dung, various plant materials like residue after harvesting the crops, vegetable waste and sewage are decomposed in the absence of oxygen to give bio gas