Name the process by which energy is produced in a nuclear reactor.

Name the agent which decomposes animal dung into biogas. Describe the steps involved in the process.

With the help of a labelled diagram,explain the construction and working of a solar water heater.

Name any two elements that are used in fabricating solar cells.

Explain why obtaining energy from nuclear fusion reactions is preferable to obtaining energy from nuclear fission reactions. Give two reasons.

Why is the use of biomass to generate heat:
$(i)$ discouraged by burning it directly in ovens (chulhas),and
$(ii)$ encouraged for obtaining biogas through a biogas plant?