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?

For producing electricity,the energy from flowing water is preferred to energy obtained by burning coke. State two reasons for it.

There are about $50$ teachers in a school and most of them come to school by their own cars. Many of the teachers come from the same place but everyone prefers to come to school by his/her own car.
Answer the following questions based on the above information:
$(a)$ Do you think that their practice of commuting to school is environmental-friendly? Give one suggestion. Justify your answer.
$(b)$ In what other two ways can we become environmental-friendly?

$A$ modern selenium solar cell can convert $25 \%$ of sunlight into electrical energy.

What is biomass? What can be done to obtain bio-energy using biomass?

Why is $CNG$ considered an environmentally friendly fuel?