Name any two renewable sources of energy.

What type of energy is utilized by a sailing boat?

Which elements are used to make a solar cell?

Mention the minimum temperature difference required between surface water and the water at depths of up to $2\, km$ in an ocean thermal energy plant. (in $^{\circ} C$)

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?

Mention the minimum wind velocity required for obtaining electric power with a windmill generator.