Write any two examples of fossil fuels.

What is biomass? Name the reaction that takes place in the biogas plant. Write the main constituents of biogas. List two reasons for considering biogas an ideal fuel for domestic use.

The fraction of the sun's energy received on the earth is about: (in $\%$)

The energy liberated by burning $1 \,g$ of charcoal is approximately: (in $,kJ$)

Choose the correct statement.

Why are the solar cookers painted black?