Which of the following fuels has the highest calorific value?

$(a)$ $A$ hydropower plant converts one form of energy into another. Name these two forms of energy.
$(b)$ State two advantages of generating hydroelectricity compared to the generation of electricity in a thermal power plant.

The use of dry wood as domestic fuel is not considered good. State two reasons for it.

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

Which one of the following forms of energy leads to least environmental pollution in the process of its harnessing and utilisation?

Write the characteristic features of the microorganisms which help in the production of biogas in a biogas plant.